Well, being in the Uk lipo options are limited, extremely limited infact unless you want to spend a small fortune in import fees (HMC&E can bite my ass).
Flightpower and Trakpower lipos are indigenous and can be found on most of the Heli and plane websites, and they have excellent specs; the prices are not so excellent though.
If you plan to import, anything Mike sells in his store is great if you just avoid the Maxamps lipos
I'd favour some 20-30c 5000mah lipos myself, but a 3906 chassis will limit your options, unless its kitted out with aftermarket parts- got some pics or a link to it some place?
Also worth bearing in mind a MMM system will eat a stock or fairly stock 3906 for breakfast...