Are the prices for these batteries too good to be true - http://www.tantronics.co.uk/acatalog...ery_Packs.html
Would they lose their voltage easily or something? I'm fine with the current output, just wondering why they're so cheap!

Don't waste your money on those. They are likely welded together (as opposed to using copper battery bars which give you the lowest resistance) and they have thin guage crappy wires along with the high resistance Tamiya connectors. If they are as bad as Venom packs, you will probably get 10 mediocre runs out of them before they fail. Get some real batt packs like the ones from GP or IB. I know you are in England but check out websites like Maxamps.com.

Thanks, I'll leave them alone then.