radiator flushing

One can only guess what the problem is or who informed your that the boiler is Ok.Have you check the pressure in the boiler don't only rely on the pressure gauge They can get stuck in a position indicating water pressure is ok.First step increase pressure to 2.5bar. From the lowest level vent rads.etc This pressure should produce air first and then water.In my experience its very unlikely that your system is blocked by sludge and requires flushing.This flushing method is used when a simple balancing of the system is all that is require

I agree with DB about the pressure test. Another thing to do is close one valve on each radiator apart from one. If that radiator gets hotter all over than it did before then you know the valves are not blocked and neither is the radiator. Do this on each one. If there are cold spots it may need new valves and/or flushing. If all radiators get hot one by one then it could potentially be that the pump is no longer producing enough pressure to circulate the water sufficiently. This is just a possibility as there are several potential reasons for the poor circulation, including air locks. If you don't trust the first diagnosis I would suggest calling someone else out. At least you are now armed with a few extra bits of information!