Hi thanks everyone!
The leak started after the air conditioning was turned on and set to LO. So that concurs with the Doctor's diagnosis!! On closer inspection the top of the HBV where there is a small oblong opening near the mushroom top of the device, looks a little wet as if coolant may have leaked out. Is this how they fail (normally)?
I have already started dismantling the car - the plenum and the inlet manifolds are off and I have fully inspected all of the heater hoses at the rear. I have identified the HBV and have ordered a replacement from autodoc (which I hope is an OK firm to use).
I can't therefore try Doctor Gollum's test as the car is a bit in pieces at the moment - guess I was a little enthusiastic to try and find a split hose or similar
I have also removed the elbow at the rear of the valley area to try and get a better view of the transfer coolant pipe which I also suspected earlier in my investigation. I can't see any signs of leakage and the way the coolant was pouring out, it could not be a small leak. It was running like a tap, so to speak.
Is there any way of testing the HBV now that I have it removed from the car? Would be very satisfying to 100% identify a failed HBV before I set about re-assembly.
Cheers and best regards to all.
MG