What's causing coolant to leak from my daughter's Ford Ka?

My daughter has a 2006 Ford Ka, bought new and with full service history by Ford. Driving recently, the "high coolant temperature" warning light came on. On her May 2013 MOT there was an "advisory" that coolant was leaking from the thermostat housing, so this was replaced with a new one, which includes a new thermostat. There also appeared to be a "death rattle" from the water pump, so this was also replaced. The engine was warmed up until the radiator fan kicked in. The warning light did not come on, so we thought everything was OK.

When my daughter drove the car later in the day, the warning light came on again. She had to drive on for a couple of minutes to get the car to a safe place. By then, coolant was leaking through the pressure cap on the expansion chamber. We went to retrieve the car and topped off the coolant and we were able to drive home without incident. We took the car on a longer run, again without incident. But it has happened again with the same result. Any ideas?

Asked on 7 September 2013 by DC, Maidstone

Answered by Honest John
I'd guess that the work on the car has left an airlock in the cooling/heating system. Get it back to the garage and ask them to purge the airlock. The problem may also be the electric valve between the engine coolant pipes and the heater matrix which is a weak spot on Kas and Fiestas.
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