I am posting this in the hope that I can save others from expense of going through what I have. If you want to just skip to last para the solution is there. Over the past month I have had the same probelms with my 2001 Focus 1.6 Zetec. I have owned the car from new and various members of the fmaily have used it so its taken 11 years for this issue to come out. The temp gauge would shoot to the red mostly when on a motorway or climbing a long hill but it gradually got worse to the extent that even on short journeys it was an issue.

Cut a long story short, change thermostat, then temp sender, then full flush of rad, then had water pump examined by camera, then garage found that head was an issue when tested at high tempertaure as gas was discoulouring test liquid they used. Head was warped probably due to overheating.

After rebuild no better so garage spend hrs on the interent to find solution (I had tried and not found one). like above everyone says they have an issue but no one says what the solution is!!! So thanks to my garage (an independent) here it is for those who feel there is no end to the problem.

Ford in 2001 issued a bulletin on overheating 1.4L and 1.6l Zetec engines. Bulletin TSB 005/2001 which was updated by bulletin no.5 dated 17th may 2001 (1 month after I bought the car). The fix involves new expansion tank and cap, new 74degree thermostat and new coolant hoses to splice into exisiting. Thanks to my indepedent garage they found this - when they spoke to Ford they knew nothing but went and investigated and parts were delivered in 24hrs once they had agreed it was the fix. Ford should be ashamed of themselves for not informing drivers better as there many many drivers out there who seem to have the issue but no one seems to know the fix. Hope this helps, my car to date has not overheated despite my best efforts in testing the garages work.