Well, I spoke too soon - I've been singing the praises of how reliable my car has been, and it decided to dump its coolant on the tesco fuel forecourt...
It was spewing out of the cap, so thought I hadn't screwed it on properly when I topped up this morning - put it back on fully - but got down the road and the temp warning light came on, so pulled over immediately and turned off.
Poured 2 litres of coolant in and it disappeared - bit of a puddle appeared under the car.
Had a few litres of water dropped off and some radweld to get me home. After topping up another 2 litres of water the leak appears to have stopped.
I had been losing a few hundred mils of water each week, but no sign of it - had Lates look at it at last service and nothing was found. Replaced cap and all seemed to be well.
Now to get it booked in for a good look at a local garage, can't risk taking it too far - really bad timing - I was going to take her to London for a run this week
Fingers crossed it'll be something simple.