I got this on Tuesday, at CANNE, from Swiss 128.05.
The lady controller demanded an immediate descent from FL180 to FL170. I refused "unable to comply due icing conditions below" as we were just above the stuff.
We never saw it, and never saw anything on TCAS (set for +/- 3000ft vertically, and with about 10-15nm range). It sounded like it was going to pass to one side of us, 4 miles away, maybe a similar direction.
There was only one occasion when ATC pushed me down to make room for a jet behind and that was Italian ATC, when entering Italy from Austria, when we were at FL190 and a Ryanair was behind at FL190. On that occasion, we were descending anyway (wx not an issue).
What are the ATC rules for pushing down slow enroute traffic to make room for a jet behind?
I thought that min separation when vectored (they could have asked me to maintain the heading) is 3nm. Also ATC ought to know that piston GA is going to be much more performance limited - 50fpm versus perhaps 3000-5000fpm for the jet.
Well, in Greece enroute radar separation is 10nm, becoming 5-3 in the TMAs with radar. I am not a radar ATCO, but it could be a mistake. Unfortunately not all ATCOs are aware of the performance of all aircraft types.
Peter, next time you can try: "Unable, peeing in a bottle right now".
I have never seen that. And I am relatively slow for the FLs I am flying at. They would always in my experience vector you laterally.
The French asked if I was pressurised. That is unusual too. I said I have oxygen.
You do sometimes get real gems from ATC. On a flight from Shoreham to Granada (Spain) I was asked what aircraft it was. The man said he has never seen a plane from the UK going that far into Spain.
What are the ATC rules for pushing down slow enroute traffic to make room for a jet behind?
As I understand, ATC in every country has a manual for conflict resolution of traffic.
I think the basics are pretty much the same but the specifics can be very different.
In the USA this manual is JO 7110.65 which you can read here.
In the UK it is MATS. Part 1 is available for the public to read while Part 2 is not, and Part 3 is specific to the airport.
You've done well by saying "unable due to icing". Some pilots blindly follow ATC instructions and kill themselves as they forget to put their own safety first.
The French asked if I was pressurised. That is unusual too. I said I have oxygen.
Funny seeing it is a French plane and all...
I have never seen that. And I am relatively slow for the FLs I am flying at. They would always in my experience vector you laterally
Same for us in a CE525A
I had one "descent immediately" encounter in my career, that was 2009 in Poland. I complied and did a 1500fpm dive and then asked why? The answer was "not you, the other German airplane, sorry, climb to FL100".
I think by now they are able to handle more than on German registered aircraft in their airspace 
Also ATC ought to know that piston GA is going to be much more performance limited - 50fpm versus perhaps 3000-5000fpm for the jet
An A320 won't be anything like that good, maybe 2000 ft/min at FL100, 1000 ft/min at FL300, and is only certified for FL390 maximum.
