If some radiators are warm then it is unlikely to be the valve - that just directs hot water from the boiler to either the hot water, heating or both (unless your heating is very advanced and has a valve between upstairs and downstairs).
Symptoms of faulty valve therefore are heating working but no hot water or vice versa.
Also, can't see how it could be 'sludge' - hot water has to go round the system to get upstairs, unless every downstairs radiator is 'gunked' but none of the upstaits ones are.
Sounds like the pump to me - upstairs radiators will get warm through convection in the heating system.
Does the pump make any noise (and did it before) i.e. can you hear it when it should be pumping water?
This is based on previous experience with my own faulty heating - not any plumbing knowledge!