Im not too sure what range of cars you have in the UK, but over here in Oz, I found a 12v 75watt pancake motor which seems to handle 24v quite nicely. It came out of a Ford Telstar. It weights 750g, 110mm diameter and 40mm tall including shaft(shaft is 8mm double D shaped).

If your looking for a non-pancake style motor to suit your design better, a Mini EV is a cheap and effective weapon motor. We have one in our featherweight, Vertical Limits, which has a 3.4kg 300mm diameter flywheel which rotates at around 4000rpm. The Mini EV survives a full battle just running of two crappy 2.3amp SLAs in parallel.

You can buy Mini Evs here: http://www.robotmarketplace.com/marketplace_motorsmisc.htmlhttp://www.robotmarketplace.com/mark...otorsmisc.html

On that link, check out the HTI motors with 5mm shafts, they might be a half decent motor, but I havent personally used any.