expansion vessel - engineer correct?

Hi there,

our worcester bosch 240 has had a new diverter valve fitted by a brit gas engineer. Following the fit the pressure went from a constant 0.7, up to 1.5. Following the fit the heating didnt work but the hot water did. He returned and fitted a new pump and auto air relief. Heating still didnt work. Returned today, replaced diverter valve, the pressure rockets up to 4! He says the expansion vessel needs replacing. I'm not satisfied with the buys integrity (replacing unneeded parts, instaling fualty parts. Could it be that the pressure issue is a result of the work he has carried out? The pressure only became an issue after his first of 3 visits?

Many thanks in advance

Jamie
March 2009