header tank overflow?

Header tank is overfilling and overflowing at night after Water and heating have been turned off. It does not do it in day when system is running.

Both ballcocks have been replaced in tanks to making sure nothing is letting by so prfesumably water is coming back from the system.
A new cylinder has been fitted and this did not solve the problem

Nowfaced with anothe mortgage( lol) for power flushing the whole system Is this likely to solve the problem.?

Any comments appreciated