Thanks for your tip Paul. A diverter valve makes sense - the leaking
part is connected to the two boiler outputs - HW and flow.

I've just checked on the web and found a picture of a Potterton
diverter valve and i'm positive that this is the part i need (only for
a Vokera!). If anyone knows where i might find this, would be
appreciated. Will phone round local suppliers tomorrow.

Incidentally, i was mistaken in my original post. The system had lost
all pressure, but once i filled it up again, it stayed filled. This
discovery means i can actually use the heating for short periods as
long as i have a bucket beneath to catch the trickle of water.

Certainly doesn't look an easy part to replace - seems to be connected
to half a dozen different pipes!

Daljit