thanks for getting back
there is not a tap on either end of the pipe. There was a blue plastic lever? over a screw on the domestic input end, which came crumbled in my hand.
I opened that screw and the one at the other end. Water came out of the "central heating return" end, but the gauge did not increase, after about 1 minute.
Should I have left it longer ? Is it right there was water?
Fifi and the kids, December 2008