My theory is that the new water pump is faulty, resulting in reduced circulation of water in the system.
What little circulation there is is gradually warming up the interior and in winter is just enough to keep the engine cool enough for short trips.
The air inside did get warmer when I was driving on the motorway, probably due to the extra cooling air through the radiator causing better water circulation.
Is this a feasible theory?
It is booked into the garage to have a look at the water pump tomorrow. So hopefully I will find out.
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