Thanks for the help and links guys. We can't really go LiPo; all plastic robot, it would mean quite some doing to get the thing LiPo safe, and we don't have the time to do that.

We're likely going to get a ton of NiMH cells. We sent an email to Component Shop earlier, asking for a quote regarding custom packs. We're looking for about an end result of 5000mah, 60a discharge, from two packs. If their price is too high we will make them ourselves.

Still researching. No point in purchasing until Monday anyway, so we have a few days to work things out. The cheapest we will be able to get the batteries will be about £60, for the raw cells alone. We can't justify another £40 on top of that for a charger without eliminating all other routes, including that of our current charger.

Thanks again guys. All suggestions are being taken in. Interesting to hear about the odd behaviour with capacity, Dave. Sounds like more or less what we have come across...

Edit: why is the pro peak so expensive here in comparison to here?