I was surprised at the question, when new central heating fitted to dads house all the old pipes were left in place. The took the boiler, header tank and cistern, but not pipes.
Father-in-law had problem with power flush, the central heating had a fault, motorised valve failed, and no radiator in living room, since there was a gas fire in living room, no radiator fitted. This work was completed, and re-filled etc, and all ran A1, but it had been a problem getting the work done in North Wales when living in mid Wales so we decided best option was to get a maintenance plan.
They sampled water and said it needed flushing, so got the plumber who had done all work before to flush it, but then the maintenance people would still not take over the boiler servicing, they said the system still needed flushing. Father-in-law died, and house sold, and still the maintenance company had not taken over the servicing in spite of taking the money. They insisted they would need to visit again, and we simply said no way, no longer our house, so did get money back.
However it does seem flushing has to be tested, not a clue what they tested, water looked clear until inhibitor went in, the slight milky look was due to the inhibitor our plumber said. As to who was connect not a clue.
I am lucky with this house still have the cistern, so in the summer we have free hot water, we nearly had it ripped out, thank god we didn't so winter DHW is heated by oil, summer it's heated by surplus solar.