Anyone know how many amps the starter motor draws when turning the engine over? Reason I ask is I want to fit a circuit breaker in the bonnet (as the battery is in the boot), now I've got a cool Aepro one at work with a built in digital voltmeter, but it's only 120amps.

The current draw would be different for differing starters, loads, etc. You should pull the starter and check it's specs. All electric motors are required to have the specs on a plate attached to them. I can't see starters being any different. What ever the rating is multiply by at least 1.2 and that is your Max current rating.

that might be the case on an ac motor but i havent seen one of these plates on a starter motor yet.

where a manufacturer will tell u that a starter motor will only pull 80-100amps they can and do spike to 200 on initial contact. depending on how sensitive your circuit breaker is it might not be big enough.

Starters will draw more current as load increases, as tara said initial draw is very high. Go and see an auto sparky or anybody with a clamp ampmeter. That way you can check what your starter will draw without guess work.