I've just replaced the diverter valve on a Potterton HE (and the motor) and its still leaking out of the top of the valve, is the new valve also faulty or am I missing something?
here is the old diverter valve, been leaking for some time and the motor was getting waterlogged, I replaced that as well because I lost a spring drying it out.
Probably really old stock and had the cartridge replaced previously. Email them and send it back, get yourself a new one from a local supplier. You get what you pay for.