Reluctant Boiler!

Our as CH has a Potterton Kingfisher Mf boiler, everything (hw , ch) is basically working but system is never able to achieve the temperature set on the thermostat. Currently its struggling to reach more than about 18 deg.
All the radiators are bled, they do get warm but not hot - you can leave your hand on them - if thats anything to go by.
The thing is, the thermostat knob on the boiler is set about halfway; if I try and turn it up even slightly (say when the boiler is on/heatin) it just cuts off. When the boiler comes on, it will run for about a minute or so and then switch off again.
The only other thing that might be relevant is that i do think there is some sludge in the system, couple of the radiators have cold areas or cold at the bottom. Is it worth buying some sludge remover from Fernox or whatever?

Sounds like circulation,pump ok?,as its not staying on long enough to reach temp,but it must be hot by the boiler if its cutting off,is it red hot at the pipes at the boiler?

The boiler thermostat (the potentiometer) could be the problem. If you have a test meter check the resistance (on the two wire connections on the front part) as you adjust it past halfway - it probably goes open circuit.

Cheap part.