In the area where the pump is I've now removed all the housing so I acn see what's going on.
The pump impeller shaft is definitely turning, but I have no way of knowing whether the impeller has come off the shaft. So, that's a possibility.
I have a pair of pipes labelled bf and br - boiler feed and return. the bf pipe has the pump on it.
The boiler feed pipe leads down to a 3 way valve. Honeywell VC4013-02. One arm leads off to the hot water cylinder. That pipe is hot. The other arm goes to the central heating flow pipe. That side is all coldish. So, I'm coming to the conclusion it's actually the motorised valve that's not working correctly.
I took the actuator off, and there's a spring loaded piston on the hydronic part which I can't move. I'm not sure how stiff this is meant to be? This part of the system is all in 28mm pipework, so it's a really chunky valve and I'm not clear how easy it ought to be to move the piston?
In case it's any help, I found a spec sheet for the valves here: