Nick,
As the fault only appeared post MOT I would be inclined to think they inadvertently disturbed some part of the cooling system or possibly the otter has taken a dive but this is readily disconnected or by passed to check it's function - I removed mine on my first car and checked it's operation out, suspended but with lower section lmmersed in water in one of the long haired Major's saucepans. You have to chose the right moment of course..
Ohmeter and her jam thermometer is all you need.
