If you stick to OEM brands like Mahle, Nissens, Valeo, Hella, VDO etc, you should be fine. I could be wrong, but I think dealers will only sell the entire fan pack, and it will be a good £300+ I reckon. Have a poke around on AutoDoc and see what they've got.
Probably not a bad idea to replace the whole lot to be fair. Saves you going back in there again when the other one fails!