Alright mate, I dunno about you but £40 is still a decent amount of money to me!
I just got the 10AH mate and it's more than enough really for what I do. I go out for 2-3 hours normally and the battery has only ever been just under half full when I get home, but I charge it up for the next time out anyway. Once I forgot to charge it and went out a few nights in a row, (only for a couple of hours max each night though), and it still lasted over an hour on the last night before it ran out.
As for where to buy - I just bought it straight from Deben. I looked around on mail-order companies