Bert .... The answer in all cases is that the plumber did.

He advised replacing the boiler, which was nearly 30 years old after the pump burnt out 3 times in less than 2 years.

He defined the scope of the job

He reconnected the system wrongly ..... the hot water always used to flow through the thermostats first.

He had sorted the boiler out in the past

He was a boiler specialist and Corgi registered

BUT he had just returned to work after an operation on his hand and (if I'm being generous) I would say that he was probably rushing to catch up on work and was not taking the care that he should.

We shall see what happens tomorrow ..... I'll let you know