In case the link above doesn't work or is removed in the future, the cure that finally resolved my cold downstairs radiator problems was to use Fernox DS40.
This stuff is magic but should be used with caution as it can eat your boiler and/or pipework. You can buy it in B+Q and comes in a large tub.
I tried about 3 different sludge removers, DIY powerflushing with a hosepipe, drained the system about half a dozen times, banging the radiators, even changing a radiator but nothing fixed it until i used DS40.
After about 10 minutes in the system, the rads downstairs started to work. hooray! They are still red hot now some 2-3 years later.
.I know this is not so relevant but do you still have your Baxi WM?
If so is it the one with the brown line in front?
I was just wondering if it was the same model as my old Baxi that lasted approximately 22 years and was still working when taken out
Yeah, that's the one. Not sure how old mine is but it's still working.
However, last week a brown puddle appeared underneath it! It's stopped leaking now but i think it's probably on it's way out. Out of interest, what new boiler did you replace it with? I was looking at Bosch Worcesters or maybe another BAXI.