Pootle wrote:I'll let you know if this fixes the problem.
I finally fixed this by buying a new coolant level sensor from Brickwerks. It was different to the part I bought from Euro Car Parts in Filton - the Brickwerks one had totally exposed electrodes, but the Euro one had electrodes enclosed in plastic except for the tips.
I was surprised because I couldn't see anything wrong with the Euro Car Parts one and it passed a conductivity test with my multimeter. I guess the Coolant Level Control Unit is very sensitive to the amount of sensor in the liquid.