Providing the prv is ok and reseating after leaking it should take no more than half hour and is a basic call out, so whatever they charge around yourway.
If it only does it when heating on that would suggest filling loop is not passing and the only other thing that would cause it is the plate heat ex passing but this would show on HW demand and cause a gradual rise over time even when not on.
Thanks i did notice that one of the two black taps on the filling loop was in the on position (I have now turned it to off)
Could this be the problem ?
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