crasslin zone port valve faulty

[toadhall]

Hi, when I switch on the underfloor heating, the entire central heating for the house comes on when they should be independent. I have figured out that one of my 2 zone port valves is faulty. Basically, the manual lever seems to work normally (i.e has resistance when moving it, and then falls back into place when left alone), but when it is in the closed position, hot water still flows through it, supplying the central heating. Is there anyway of telling whether it is the metal valve that is at fault or the electric motor/mechanism that sits on top?

Thanks
t