The best offer for the plug from an aviation outfit is approx 80 EUR.
And with 24 V and starter currents I would not take the standard jumper cables… unless in wintere against cold hands ;-)
Checked the caravan shops but could not find that plug – they have usually the three pin in blue.
… and my real question was not – how can I save a few bucks quid 
, but rather
how often have you been in need of external power in recent years?
With the Cirrus i have not used EP for engine start in 4.5 years – and i won’t, because using EP with an empty battery can destroy the electrical system and/or avionics. The only acceptable method is to remove the battery and charge it. But i never had to, always starts on one or two blades (of four :-))
OTOH i did occasionally use EP to start my Piper Warrior in very low temperatures. But that was only necessary on very cold winter days after not having flown for two weeks. But i would also not do it if the battery was completely discharged.
This is what I have
I needed external power just once, about 10 years ago, at the now-closed Plymouth airport, when I left the Master switch ON and by the time I got back a few hours later the battery was almost flat.
Currently I don’t carry anything for this purpose.
There is a procedure for starting with external power which has to be followed. This is because, on most (but not all) types the external power connection disconnects the battery, so when the engine is running, the alternator / voltage regulator system will not generate a stable voltage (once the external power unit is unplugged) and could blow up the avionics. On the TB20GT, connecting external power disconnects the battery, so when you get the engine running, you do NOT want to power the avionics until the external power has been removed and the Master is back on to reconnect the battery, to stabilise the bus voltage. There is a procedure, which for the TB20 is something like
Battery Master Off
Connect Plug/Apply Power
Start Engine
Disconnect Plug
Battery Master On
My take on this is that you should never jumpstart using external power because it either means that your battery is already shot, or that you will ruin it jumpstarting.
I also (think) that it is a bad idea to let the aircraft charge an empty or almost empty battery because these batteries (hearsay) “should not be charged with more than 5 Amps for a longer period of time, becasue that will either destroy them right aways or at least shorten their life expectancy”.
Does that make sense? Whatever, with the Cirrus I don’t it, because I never need it. I always have the strobes on, this way I catch it if the Master Switch was left on …
My take on this is that you should never jumpstart using external power because it either means that your battery is already shot, or that you will ruin it jumpstarting.
That is type specific.
Does that make sense?
No, because a lead acid charger will output only about 28V so the current with naturally ie down as the battery is charged.
It was Mike Busch who wrote that (on COPA):
Not only that, but GPU-starting an aircraft with a dead (or nearly dead) battery and then allowing the charging system to charge it is almost guaranteed to fry the battery, and either cause it to fail outright or at least truncate its life by a year or two. These batteries cannot tolerate being charged at more than 5 amps for any extended length of time without overheating and sustaining internal damage. The aircraft charging system has no current limiting.
Bottom line is that if you have a dead battery, the right thing to do is to connect it to a low-amp battery charger and then go have a leisurely lunch. If you absolutely MUST jump-start the airplane, then idle the engine for at least 15 or 20 minutes before advancing the throttle (but even this should be considered a last resort).
Well, that exact text is correct.
You can monitor the charging current, by looking at the ammeter.
One problem is that on some aircraft the GPU connection does not disconnect the battery, which will then get a heavy charge.
I thought so.
