Well I hope it's sorted ...
Been ok for 2 days now.
Was a complete nightmare.
3 BG visits and Dynarod visit.
Was dreaming heating lol
Turned out cause was a dripping Ballcock !!!
This caused Hot water to run Hot for 20 seconds and then coolish.
Cylinder full of hot water ...
Boiler working fine...
Whenever BG turned up , problem went away.
Turned out (3rd BG man fixed it)....
Vent pipe to hot water was just above water line in tank.
As tank filled up over night due to dripping ball cock , the pipe was went under water.
Thus in morning , water was being sucked into hot water pipes from loft tank.
As water level went down ...toilet flushes etc ..
Pipe no longer in water and start to work as normal.
Thus BG turn up and all ok Grrrrrrrrrrrr
Pipe now repositioned and dripping ball cock fixed by Dynorod and it seems ok for last two days.
BG changed Pump and cylinder thermostat.
They were then blaming it on shower mixer ...so I isolated it and it was still broken ...
Anyway been ok for two days now :-)
Fingers crossed