Driving along and the noticed heater wasn't working. My wife then remembered it was working intermittently the day before (but forgot to tell me). About 6 miles later I got a High Engine Temp warning on the satnav display, suggesting I slow down and contact the dealer. Pulled in to a garage and looked at the coolant plastic tank which was empty, so obviously a leak somewhere but couldn't see anything under the car.

Booked it in to dealer for 3 days time and continued to drive it over the next couple of days and twice put approx 1.5 litres into the water tank.

It's now at the dealer (Hexagon, London) and after 2 days they can't find a leak anywhere and say it's not loosing any coolant, even though they have pressurised the system and taken it out on long runs. I've suggested they keep it for a third day, but they clearly think I'm imagining it.

Anyone got any suggestions?

I experienced this problem. I always check the water and oil when I wash the car.
One day the level in the plastic container was low. I topped it up with antifreeze water mix. A week or so the same low level. I began marking the level each week , same time same place, with a black marker on the outside of the container.
Car went back to the main dealer, Wellsway, Bath, 9 times. Each time they fixed the problem before giving it back but each time the problem remained. Eventually I took the car to a different MINI dealer. On the second visit they decided that that the problem lay with the head gasket, despite the first dealer so say having changed it.
This time the problem was solved and we had not further water loss.
This was carried out under warranty ( must have cost MINI a lot of money) Problem started when the car was 18 months old

Hope this helps
Kind regards
Jeff

Had same problem, even though new radiator had been put on, still losing coolant then my brother said he thought it could be thermostat housing, sure as eggs r eggs it was and the garage fit a new one, not lost any water since, common fault on mini apparently .hope this may help