I've driven 240 miles in the 850 this week and glad to report everything is perfect, air-con is icy cold and the engine temperature gauge sits bang in the middle. I checked the water level every time I was out expecting to have to top it up after giving it a full coolant flush but the level hasn't moved which means I've cured the leak at the waterpump so I can take out the can of coolant I've been lugging around. Happy days
EDIT: I mentioned there was a problem with the ride a few posts up which I've got to the bottom of. Turned out 'National Tyres' fitted two different sizes of tyres to the car. I ordered four of 205/50R16 and they fitted two that size and two 205/55R16. I also had them check the balancing and every single wheel needed re-balanced. I won't be buying tyres from them again...
