I bought a new 7.5 amp hour the other day. £12.50 from a local supplier and it's made by Lucas.
They had another Lucas battery rated at 10.5 amps at around £17.00.
This 10.5 amp will just fit into the backpack without the soft black plastic battery cover, the hard cover will still go on. This should last about 30% longer than the standard one supplied. Dai