WHOOOOOO HOOOOO
I've solved the mystery of the lost coolant!!
I decided to look at it in my lunchbreak today. I looked and felt all around the hoses/coolant connections, RAD, HBV, looked for any signs of it coming out the water pump... even held my hand around the HBV with the engine running....
However, I wiggled the auxiliary coolant pump, and smelt Antifreeze.... I wiggled it again, and saw it dripping!
So - I've got a little split in the pipe just under the Aux coolant pump.
PHEW!
Now I've sorted that and I'm happy it's nothing terminal - I'm going to give her the much overdue cambelt/water pump change, to ensure it keeps me going for the next 10 months of pre-facelift driving.
(According to the history, this cambelt hasn't been changed in over 70k - and the Aux belt is the old type (with the lower aux pulley) and is split to buggery - so I can quite believe it.
Talk about borrowed time .. I'm going to order a kit and watch the DVD once more... and get the toolbox out