You can get 4 x 1000mAh rechargeable batteries - AND a USB charger (takes 4x AA or 4xAAA) for less than the price of the batteries alone (which cost £4.99 as the lowest price on Google Products - compared to £3.99).
Play.com are selling the same battery and charger set for £6.99 - so 57% of that price here.
Why not get four sets to avoid paying postage and use your match voucher to drop it to £13.46 for 16 batteries and four chargers (enough to encourage a friend to start using them perhaps, with one of your extra chargers?)
Or handy on its own to get you over the fifteen quid free postage threshold.
The £13.46 price with delivery gets you enough AAA batteries to last the next five years and compares well to the 'best price on google' of twenty quid excluding delivery (from Misco).