you might want to check the piston valve, if its cracked (it only takes a small hairline fracture to cause a leak) replace it, they're only about £1 or at worst replace the whole ball cock arm mechanism (about a tener)
I bought an electric towel radiator from screwfix, and a 250W element. The question is, with what do I fill the radiator with?? and secondly how do I control the temp? with an external timer switch outside the bathroom?