Turn boiler off at mains.
Set timer for constant heating and room stat to max.
Take out the big chrome screw in the middle of the pump.
Water should start dripping out.
Set pump to 1.
Turn mains back on, pump should start spinning fairly fast.
Set pump to 2, followed by 3. Pump speed should increase noticeably each time. Turn mains off, and replace screw.
This bleeds the pump but it may not be pumping effectively. It could be blocked, the rotor may have come off, the shaft may have broken or jammed.
The only way to be sure is to separate the halves of the pump (Allen screws) and check that the rotor is hard to stop with say one finger.
Pump valves can also become clogged.