UK Ford Repair Questions? Ask a Mechanic for Answers ASAP
Hello, I hope I can offer you some assistance.
From your description this will be due to the impeller on the water pump coming adrift. The impeller is plastic and when it comes adrift the water pump shaft rotates but the impeller doesn't which means no coolant circulation. The water pump is driven by the timing belt so is behind the timing belt cover when replaced a new timing belt is required .
The reason the engine doesn't over heat at idle is that modern diesel engines are very thermally efficient so not much heat is produced at idle, only when there is a load on the engine does it start to produce more heat.
You are correct in that this is quite a common concern on this engine.
Let me know if I can assist further and I'll reply as soon as I can.