I fitted a new radiator last autumn with a new TRV and noticed the other day the TRV has started leaking. It is leaking out of the top of the valve where the 'head' sits not from the pipe connection or the radiator 'tail' connection. It leaks only slightly, but consistently, unless the head is taken off and left off - so that the valve is effectively fully open. Is there a way of tightening something up to stop it leaking or is it likely that this is a faulty valve?