I don't think the house owner is pulling a fast one because he seems
like a decent chap.

I didn't do any pipework near the pump. The pump is located under the
bath and I was working only as far as the top of the storage tank
cupboard so there is no chance the pipework could have been affected
anywhere near the pump.

Regards the airlock, it may well have been caused by the houseowner
because as I had just turned the water to the tank back on he rushed
to check if the power shower worked. He claims to have removed the air
lock by sucking on the shower head and basin mixer tap. How you suck a
shower head I don't know.

The only reason the pump should carry on working (as far as I know) is
if there is a leak between the pump and the shower head or a faulty
sensor as you say. There's no evidence of a leak and it just seems to
coincidental for the sensor to have failed at the same time.