Brian,
I had a similar problem with a Rover 420GSI Turbo. The 2nd day that I had it, it blew a hose off to the header tank whilst driving along a single track lane so was forced to trickle along for a mile or so before I could park up. The garage where I bought it recovered the car and checked it for any damage but all seemed fine. A couple of months later it dumped its coolant. Again they checked it thoroughly, cylinder pressures fine, no sign of exhaust gasses in the coolant header tank. A few weeks later, just days before we were going to drive it down to the Pyrenees, it did it again. This time they had the head off and there was the tiniest mark of a leak between a cylinder and the water jacket. The head was skimmed and I had no more trouble for the next 4 years when I replaced it with the Kappa. We went on holiday in a new but very boring Peugeot hire car.
Peter