Sounds like an earth fault to me which then shorted through the throttle cable making it stick.
Perhaps when you switched on the screen the poor earth was overloaded which caused a volt drop at the pump cut off solenoid causing it to close part way hence the power loss
Looks like you were right actually
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Ive got it back from the garage and they sorted the badly wired switch. But when dropping off the Defender with them, they did suggest exactly what you said, and it seems you were correct, they diagnosed a kinked accelerator cable which has now been replaced.
So, it would appear...
1) Faulty switch caused volt drop at fuel pump cut off solenoid.
2) Pump solenoid partially closed, resulting in loss of power
3) Faulty switch caused a short through accelerator cable.
4) Cable was damaged as a result, causing revs to stay on.
So, when the heated windscreen switched itself off (via timer) everything could return to normal (except accelerator cable)?
When the engine cut out, was it because the fuel pump solenoid had partially closed? Was this the reason it didnt restart straight away?