I'd also recommend 7DayShop for batteries, chargers and all the other gadgety stuff that they sell. If you can bulk up the order - with others if necessary - it can help spread the £3.95 P&P fee over a larger total order price.
I ended up getting my batteries in Poundworld on Moore Street.
4 size D batteries (the big ones) cost €2.
The brand was Memorex and seem to be resonabley good.