Maplins do some very good nickel metal hydrides of their own brand. They do them at a special price from time to time - worth keeping an eye on their web site.
Cheap NiMhs tend to fail prematurely: avoid them.
Oh yes, and get a good charger - NiMhs are sensitive to unintelligent chargers that just pump in a fixed current indefinitely. You need a clever charger that senses the batteries' temperature and state of charge and tails off the charging current to suit if you want your batteries to last well.