Sorry forgot to mention:

Petrol 1.6 manual (2003 model). Problem occurs when driving and when stationary. Now limited to using car for short journeys, i.e. stop before boiling point.

This is a great old car - hard to find a cheap replacement. I suspect this fix will be beyond my expertise and/or meagre ration of free time, so I will probably have to invest in local garage support. But it would be good to know what I am likely to be hit with!

Many thanks for your time and advice :-)

Thank you so much for offering advice and wisdom - it's really heartening to be helped by strangers - restores one faith in mankind with all the s*** that seems to surround us sometimes ...

So good news - the problem is solved! And what's more it was indeed the expansion tank filler cap, making this possibly one of the cheapest repairs ever at £6.99.

Ran up the engine. At 102 degrees the coolant rose nearly up to the cap, so fearing the previous boiling out, I switched the engine off and the fan kicked in immediately as before. The coolant level dropped quickly - I restarted the engine straight away and the fan stopped immediately. The temperature had dropped to 99 in that short time!

I repeated the above with exactly the same results, suggesting that this feature is indeed an intentional precautionary measure when the car engine is turned off above a certain temperature (the fan was not activated when turning off the engine at 96, for example).

Then I decided to hold my nerve and wait until the engine hit 105. The coolant level was pretty high by this time, but the fan did kick in at 105 and ran for about 15 seconds. The coolant level dropped quickly and the temperature settled at 96. I kept the engine running and let the temperature rise to 105 again, with exactly the same results. So that cycle was repeating nicely, up to 105 then down to 96 with the fan kicking in for approx 15 seconds in each cycle.

This really is a great car, still strong and young at heart after 21 years and about 120,000 miles (the speedo drops out sometimes so not all the miles get registered - can tell the speed from the rev counter so it's not a big deal). We have had this car for 6 years and I want to keep it for ever :-)

As an aside, no response form the fan at all when switching on the aircon. It did not work when we bought the car 6 years ago, so maybe beyond help.

Finally, many thanks again for your help - I wish I could buy you all a drink - but instead will obviously do my best to help others whenever I can :-)

I filled up to just below MAX. The coolant rose well above MAX at its hottest, nearly reaching the filler cap, but I do not think it was boiling despite being over 100. It definitely boiled out of the tank with the old cap - loads of fluid lost, and maybe air got into the system when refilling the tank which then looked pretty much empty. I did bleed the radiator a few test runs ago, and could do that again. Otherwise, can you suggest how I might locate or release another airlock? Many thanks for the follow up :-)

The Ford 2 litre Zetec is a doddle to fill, pretty much self bleeds. Would not have expected Ford to design a system that has multiple bleed points but the OP should be able to tell us simply by looking.

But from memory the elephant in the room was the cheap plastic thermostat housing that replaced the earlier robust solid Bakelite type material and before that cast alloy. The plastic ones crack and dump coolant.