Most of the Energiser batteries are on special offer at Asda:
Ultra+ Pack of 4 AA size ~ £2.62 each or 2 packs for £3 (save 2.24)
Ultra+ Pack of 8 AA size ~ £4.47 each or 2 packs for £5 (save 3.94)
I think other sizes (AAA, C and D) were the same deal.
I always keep a few spare packs in the cupboard so I stocked up this morning!