Part is apparently part number 1351K1, which in laymens terms is the pipe which comes out the block just behind the thermostat, runs around the engine, and into (or out of, I'm not sure which direction!) the heater matrix. Another pipe coems out of the matrix next to it. This has the top bleed screw on. I think it's one piece of pipe.
Some (bad) pictures I took



If anyone does have one they would be willing to part with, I would be very grateful. Wycombe Citroen last had one on their books in 2004, so no point asking around more dealers.
Cheers
Sam