About 2 weeks ago whilst sitting in traffic at a temporary road works the 'Warning - over heating ' message appeared on the gauges, accompanied by the coolant fans running at full speed.
I rapidly drove the car off the road and immediately shut the engine off, knowing that soon after that catastrophic engine damage could result if not attended to.
The fans continued to run as I opened the bonnet expecting the coolant level in the expansion tank to be considerably below the minimum as reported in other members experience, curiously the level was on the maximum mark and no apparent leaks at all.
After leaving the engine to cool off for a couple of hours and removing the engine top cover to aid cooling I managed to drive the 90 miles home for further investigation, luckily without the overheating warning but with fans continuing to run at full speed.
After taking off the thermostat attachment and looking into the main housing itself, it became obvious that the plastic seat of the thermostat had a piece broken off and now missing presumably in the water pump area. Fortunately I had the correct part to hand, knowing that that this might happen at some point and after a couple of hours of disassembly I found the missing piece resting just above the pump impeller itself, I managed to retrieve it before it could do any damage to the impeller.
Assembly was straightforward and because I hadn't drained the coolant down it only took about a litre to bleed and then top up, I have run the car for a few days with fans not kicking in and no loss of coolant or warning messages displayed.
In conclusion it would appear that broken piece was somehow restricting the coolant flow around the system and only causing an overheating condition when in slow moving traffic in summer temperatures. The thermostat housing in particular seems to be a weak point on these cars and now the car is 17 years old it would be prudent to change it as a matter of regular maintenance.
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Paul