Not many places selling these online but they are standard NiMh 2450mAh AA batteries with C or D cell adaptors. Reduced in the bargain section in Northampton (Sixfields) marked as 'discontinued' so there's a fair chance there will be some nationwide - a few left in Northampton too (I couldn't buy them all!).
OK, so they aren't HYBRIO or ENLOOP but they are only £2.50 for 4 with free adaptors!!!
Also had LOADS of 9v batteries too for the same price (£1.25 each).