Thank you for digging Dave.
I have just been to the car and lifted the bonnet.
That's enough work for today thank you. The amp is screwed to a substantial heat sink. So it is worth having the spare amp ready mounted on a spare heat sink....doing the job properly being prepared.
Never having changed an amp heatsink I don't know about the screws holding the heat sink to the wing. They don't look easy to get at. Are they in to captive nuts which might fail? Is serious cleaning necessary before re-fitting a heat sink to the wing or do the job again later after a roadside fitment? Will it be raining or snowing at the roadside? Will plod be rushing one along to clear the highway? Is it worth debating?
The point
relevant to the 240 is, it is worth having the spare amp mounted to a spare heat sink. Saves possibly polishing and applying paste at the roadside in possibly adverse conditions or circumstance. Have I said before, be prepared.
I am curious about the fitment of the heat sink to the wing now. Clifford Pope is one of life and Volvo owner good eggs. As indeed are you Dave. I'll pick his brains on this for my own preparedness.
Moto re this: there is more than one good proper way to skin a rabbit. 
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