I know it's a very late response but just for the sake of any future Googlers. It turned out to be a pump. I have called in another "gas man" and he replaced the pump. This time it worked.
Well hate to say I told you so but if you had read all my posts you would have seen that you had the wrong pump head installed, not your fault it was your repair engineer that made an erse of it the Grundfoss or Bosch will fit in but wont be able to turn when power is applied as the impellor is too big
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