Weird central heating issues

Evening all. I am just trying to get some idea as had a couple of plumbers round but still getting same issue . Basically moved house recently and i just cant get figure the central heating system out .So.... Its had a new Worcester bosch boiler about 2 years ago so all good on that front and heating works well and all rads get roasting hot..until.... When the hot water comes on rads seem to cool down and about once a week one doesnt even heat up ( its the furthest away ) . It done it again this eve. Again switched hot water off. Big gurgle and within seconds all rads roasting hot again. Ive looked at the motorised valve and it seems to be moving but perhaps not enough.. any help would be great. Thanks guys

Some clear pictures of what you have would be useful
Boiler - pump if separate to boiler motorised valves pics of pipework where they are.

Ok sorry all ive been at work and not had chance to reply. So its a Worcester bosch boiler greenstar which js downstairs. The hot water cylinder, 3 way motorised valve and grundfoss pump are all in the airing cupboard upstairs all very close together. As for the pipework its all 15mm up stairs and 10mm microbore downstairs. I know it doesnt make a huge difference as they are bidirectional but noticed some of the TRVs have been put on the return not the flow aswell. Found out it new cylinder and boiler was all installed by British gas about 3 to 4 years ago by my nextdoor neighbour as the house was a probate house when we brought it
Ive attached a photo aswell. Hipe that gives you some idea
Thanks all

Please use your original thread