Decision made, just got the Hero2 motorsport edition for £179 - plus Argos kindly emailed a £5 discount voucher this morning which was nice of them. Man maths mean that allowing for voucher and credit card cash back it cost me £173 so can't argue with that.
In case it is of benefit to others, the reasons for going for the older Hero 2 are:
a) came with an in car mount which the Hero 3 Silver didn't (that is about £23 on its own) and I want to use that.
b) full size SD cards in the Hero 2 - same as my current camera and camcorder. Micro SD's in the Hero 3.
c) as they can both be programmed with one button start/stop I can live without the wifi
d) the Hero 3 silver like for like with the mount would be about £250, so difficult to justify the extra
The Hero 3 Silver is better in a few ways though:
-better sound for some things (although loses some base so maybe not if it is just recording exhaust notes)
-lower wind noise
-slightly smaller
-wifi if you can use it
-marginally better image quality - debatable in the dry, but noticeable if you want to use it under water (I don't).
Hopefully that summary may help someone, and thanks all for the earlier comments.