Motors:
I’ve head lots of people mentioning they are trying these scooter motors out, and was wondering on the verdict. Are they up to scratch? how does the actual power compare to other better known motors? And has anyone opened them up and seen if there’s anything internally that can be done to toughen them up?
Aftershock used the 24V 800W versions in Robot Wars and said they were great. I've recently bought them to replace the 750s in Coyote and so far, so good. I haven't given them a proper test drive, just a cautious run on the workshop floor (which isn't huge) but they seem to have plenty of grunt. I've opened mine up to add epoxy to the magnets to help hold them in place and to replace the stock wire with larger gauge stuff. Getting the endcap back on afterwards is tricky though but the general build quality is good.

Ragebridge 2 speed controllers:
when people refer to them being used are they normally referring to them being used in single channel mode, or would a single controller cope in with both drive motors in dual mode. (they will be fitted with a large heat sink and fan, and then shock mounted). And i've only found them on the robot marketplace, has anyone found a uk supplier, or is the RMP the only option (if so what kind of delivery cost am I expecting)
I'd recommend using one for each side, I personally haven't used them but understand they might not be beefy enough to cope with two 800-1000W motors in dual mode. The Ragebridge is just made by Charles Guan from the Overhaul Battlebots team as opposed to a large company with European distributors, so as far as I know, RMP is the only option at the moment. Hard to say on delivery cost; throw them in your basket and go through checkout (but not completing the payment) to see how much it tacks on.

Chain size:
I had been planning on using half inch chain, but the scooter motors mostly seem to come already with 8mm sprockets, would 8mm chain be a viable option instead, or is it a bit lightweight for this application?
I personally feel 8mm chain is a bit on the small side for HW drive applications, but I do know of some people who are planning on sticking with it. I have 1/2" drive chains on Coyote but in contrast, they feel a little overkill. Might be worth switching to 3/8" chain for a good middle ground; I think this is what most heavyweights that aren't designed for pushing power use for their transmissions.