I've not tried bypassing the circuit, I'll give it a go tomorrow but I doubt that was the issue as the same thing happens with the replacement switch. I connected the original switch to a multimeter and it appeared to be working but I'd already ordered the replacement at that point. When I turn the key it's exactly like what would happen when you start the car without depressing the clutch, the electrics all switch on and that's it. The starter motor isn't turning over until the engine eventually fires and once the starter engages there's no problem. I wondered if it was the immobiliser but the NATS light turns off when the key is in the on position. Everything suggests it's the clutch switch but I just can't get that to work so I'm completely stumped.