I've got a couple of TRV4 valves which are leaking a bit around the
pin. Google has revealed a YouTube video

http://www.youtube.com/watch?v=cC_HokOML1A

which, towards the end, suggests that a gland seal replacement kit is
available, enabling you to replace the seal without draining the
system.

I've tried googling for the kit, but apart from a few mentions that it
exists I can't find out a source them. Anyone know where I might get
them from?