UK stores recommend the 7.4 or 8.4v, US stores recommend 8.4v or 9.6v - and a few sites claim 11.1v compatibility.
Going with:
http://www.landwarriorairsoft.com/batteries-bbs-gas-chronographs-c5/batteries-c62/we-airsoft-europe-7-4v-2600mah-20c-crane-stock-lipo-p3197
http://www.landwarriorairsoft.com/batteries-bbs-gas-chronographs-c5/batteries-c62/we-airsoft-europe-7-4v-1450mah-25c-crane-stock-lipo-p3193
Having just noticed cells are different in LiPOs.
17/07/2015:
Well, I bought it from Land Warrior Airsoft yesterday and received it today. Price matched a £1 off too.
While I have had no surprises, it is still managing to be lower quality than I expected (for £154 item manufactured outside the UK). I would not call anything I dislike about it 'unexpected' and I am quite happy with it - not fired yet. G&Gs manufacturing tolerances between the parts seems to be lax, but the parts themselves seem individually fine, small amount of rattle, but it is quite lightweight, and overall I don't have any regrets at this point.
Well, except that I have foolishly not ordered a sling.
Shame it does not have ambi mag release.