Thanks Stevo,

I've ruled out the starter motor for two reasons. Firstly it always works from cold and secondly it was working fine right up to the engine switchover. No noticeable change in cranking speed/sound.

I had my battery tested and it was found to be bad. Thinking I had hit the jackpot, I exchanged it for a brand new one under warranty and fitted it with great expectation. The result = No difference!

We've also tried both crankshaft sensors (from both cars) and it doesn't make any difference.

We even changed that complete section of wiring loom for the one on the other car and yes, IT MADE NO DIFFERENCE!!