Valliant boilers are probably the easiest to fault find, they have everything you need at your finger tips, they incorporate:
find an engineer who knows how to access and interrupt these.
No point guessing at the power flushing either, ask for the following tests and readings:
These test will high light the system water quality and confirm if a flush is required or not. (in some extreme cases you may need to send a sample off for more detailed info).
here is a sta.b in the dark hope it helps you out:
Their should be what we call a boat filter located at the base of the flow pipe on the left hand side of the boiler, located inside a brass housing, it can be a pain to remove but I am guessing (considering we have no data to go on) its full of muck, specially if a power flush has been carried out.
I hope that was of some help, all the best.