This issue stumped me a little on my 2001 XK8, even after searching on the forum for while, so I thought I would post a quick note.
The previous owner had picked up the car as a low mileage 'bargain' with a problem engine. After replacing the engine due to a spun bearing, and numerous other issues, he had had enough and I snapped it up. Symptoms were the belt falling off and radiator top hose coming off the thermostat housing. For some reason I thought the belt was yanking the radiator hose off when it came loose, although I'm not sure how I convinced myself that was the problem. Wiser heads would have guessed that the problem was the plastic thermostat housing, where the ridged end of the plastic pipe that attaches to the top radiator hose was snapped off. There was plenty of fitting left to clamp the hose to, but with the ridged end gone, as soon as enough pressure built up the hose would pop off, soaking the belt, which would then come loose. Instant loss of power steering and coolant all over the road. Changing out the housing to a replacement metal one and all was well, once I realised that there should have been a ridge there in the first place. I assume that the housing was probably damaged when the XJ8 replacement engine was pulled from the donor car by the breaker, and not noticed when installed by the previous owner. It was a nice clean break, no jagged edges to advertise the problem.

 

I can understand how difficult it was to work out the cause with the broken original plastic housing!



I've added RESOLVED to your thread title to help anyone with the same issue in future.

Graham