My understanding is that the answer is ALWAYS unless being vectored by ATC.
But aren't there some potentially nasty situations e.g. when ATC says "cleared for the approach"? In that case you are entitled to immediately descend (at an unlimited rate of descent) to the platform altitude of the approach you have been cleared for, and fly the approach all the way to the runway (but you cannot touch down; for that you need "cleared to land").
What is the procedure (if any) which an ATCO needs to follow in such a case? Does he/she have to wait for the aircraft to be obstacle-safe before speaking that phrase?
According to my instructor, cleared for the approach is one of those dangerous scenarios. One might assume that this means you are terrain-safe, but that is not so. The only time ATC is responsible for terrain separation is when you are under vectors, as you say. Therefore, although you are cleared to descend, you still need to use the sector safe altitudes on the plate until the IAF.
Here in the US, a clearance for the approach does not permit descent until one is on a published segment of the procedure or airway. If the aircraft is not on a published segment of the approach at the time the clearance for the approach is issued, ATC is required to issue an altitude to maintain until on a published segment.
I don't think you can descend until you are on the approach. ie, at EGTK on the NDB/ILS, you can't descend below MSA until you are outbound on the procedure. Now in practice ATC will have dropped you to 3500 anyway.
In that case you are entitled to immediately descend (at an unlimited rate of descent) to the platform altitude of the approach you have been cleared for,
no longer the case under EASA rules! However; the approach designer has ensured that if you follow the published procedure, you will have adequate terrain separation. It doesn't matter when the ATCO says you are cleared the procedure, as it always starts from a defined point. The ATCO's job is to separate you from other traffic and advise you if he observes you going outside the domain.
I recall an approach into Birmingham where my student was well below the profile at which point the ATCO said "you are responsible for your own terrain clearance" i.e. he had observed the aircraft outside the defined domain of the approach. The pilot is always responsible for terrain clearance.
Well, if you are cleared for the approach, you have to follow the approach exactly as shown on the plate, which means descend to the correct altitudes etc. If unable, e.g. if you are too high and/or feel uncomfortable with the situation, you request to hold to lose the altitude to begin the approach. Cleared for the approach too early doesn't mean that you are cleared to descend immediately all the way down (e.g. because you want to make a nice shallow descend from far away), because you may not know the airspace structure and the Air Traffic Services provided in that airspace, e.g. you may fly into uncontrolled airspace... So, just follow the chart.
If vectored, the ATCO is responsible, correct, see Doc 4444 8.6.5 Vectoring.
8.6.5.2 When vectoring an IFR flight and when giving an IFR flight a direct routing which takes the aircraft off an ATS route, the controller shall issue clearances such that the prescribed obstacle clearance will exist at all times until the aircraft reaches the point where the pilot will resume own navigation. When necessary, the relevant minimum vectoring altitude shall include a correction for low temperature effect. Note 1.— When an IFR flight is being vectored, the pilot may be unable to determine the aircraft’s exact position in respect to obstacles in this area and consequently the altitude which provides the required obstacle clearance. Detailed obstacle clearance criteria are contained in PANS-OPS (Doc 8168), Volumes I and II. See also 8.6.8.2. Note 2.— It is the responsibility of the ATS authority to provide the controller with minimum altitudes corrected for temperature effect.
Now, when you are on final, you have to receive an approval of some kind in order to land. Since the ATCO must maintain a continuous watch of the traffic in the aerodrome area to ensure safety etc.
(see 7.1.1.2 Aerodrome controllers shall maintain a continuous watch on all flight operations on and in the vicinity of an aerodrome as well as vehicles and personnel on the manoeuvring area. Watch shall be maintained by visual observation, augmented in low visibility conditions by an ATS surveillance system when available.)
he will normally tell you that you're in sight and that you are cleared to land. Exception is when he has some sort of surveillance equipment that shows him your position, so in that case he may not have to actually see you through the tower windows. But if you are landing at an airport where no such surveillance equipment is present and the ATCO cannot see you (e.g. due to weather), then in my opinion he must tell you that you are "not in sight" and to "report on the ground", or that "runway is clear, report on the ground". In my opinion he must not give you a "cleared to land" because he cannot ensure that you are "safe" in these last 3-4 miles of your approach.
Cleared for the approach too early doesn't mean that you are cleared to descend immediately all the way down (e.g. because you want to make a nice shallow descend from far away), because you may not know the airspace structure and the Air Traffic Services provided in that airspace, e.g. you may fly into uncontrolled airspace
However, isn't there an ambiguity there?
Take the standard Jepp approach plate.
It shows a 25nm radius circle depicting the MSA within a 25nm radius of the airport.
Say you are 24nm out, FL150, and "cleared for the approach" whose platform is 3000ft. And the MSA in that segment is 6000ft.
To me, this means I can descend to 8000ft (6000 plus 2000, anything over 5000 is a "mountanious area" so you add 2k instead of the usual 1k) and fly to the holding fix, enter the hold, descend in the hold to 3000ft, and fly the approach all the way to the runway (but not touch the tarmac).
Or are you saying I should fly to the holding fix at FL150 and descend in the hold from FL150 to 3000ft?
I recall reading some UK CAA doc saying that the UK had for many years not used "cleared for the approach" phrase because it entitles a pilot initiated descent, which they didn't want. But they are going back to that phrase now.
Also I don't think I need to make any radio calls (unless requested) because this is the only possible way to do it. There is no way to descend to 3000ft with an obstacle clearance assurance other than in the holding pattern.
Personally I would make a call "Nxxxx taking up the hold and descending in the hold to 3000ft".
Obviously in practice traffic is managed more tightly, and in a radar environment it tends to be managed totally so the "cleared for the approach" is a meaningless formality except that without it you must fly through the localiser if thus vectored.
you have to follow the approach exactly as shown on the plate
Most aerodromes don't have plates for visual approaches, so which plate do you take?
Your view doesn't seem to be shared by many cost conscious airlines, which request visual approaches and departures to be able to short-cut the procedure.
you may not know the airspace structure and the Air Traffic Services provided in that airspace
Why is that a problem? If you agree to a visual approach, you assume all responsibility for separation to terrain and other aircraft
Your view doesn't seem to be shared by many cost conscious airlines, which request visual approaches and departures to be able to short-cut the procedure.
If you're cleared for a visual approach, then you no longer have to maintain MSA + 1000.
However, isn't there an ambiguity there?
Why? You described the procedure correctly. When you're cleared for the approach, you descend according to the procedure which means you don't go below MSA + 1000/2000 until you are on the procedure and are able to descend further. The holding pattern is the classic method for that and it will contain a MHA number next to it.
However, if you really have to fly to the hold at MSA+2000 and then descend there, then you're probably in Africa. 
Dare I ask why you would maintain MSA +1000/2000?
MSA already has that margin built into it. I would probably descend to the MSA rounded up to the next 1000' initially. One is most likely to get this sort of stuff in the "procedural" environments such as Poland where you tend to fly the full procedural arrival/approach. The STAR however normally leads you onto the approach quite nicely.
I appreciate this is more of an issue when arriving at airfields without STARs - Perpignan comes to mind, where the arrivals are all direct from en-route waypoints to the IAF and there are some rather big lumps of granite in the vicinity (up to 9000' AFE within 20NM)
Under vectors, ATC are obviously "responsible" for your terrain clearance, but they are capable of howlers - giving you someone else's vector, turning you the wrong way &c &c, so in high terrain areas I would be on my guard, and eyeballing the min vector altitudes chart and the terrain display.
