Try Euro Car Parts, and use BATTERY15 in promo code box, for 15% off.
Yeah, definately use Euro car parts, i got a battery off them Yesterday (Thur), but use the code BRISK25 for 25% off the battery and anything else on their site that ya fancy.
The only problem with Ebay is that if anything goes wrong you have to usually arrange a return courier yourself. At least with Euro they have a good RMA service or just take it back to the store for exchange.
If you know someone with a halfords trade card they are reasonable then as they are quality batteries too. If not there the auto electrician on the corner of burrowdale road in moreton are good too