Worcester 24i Combi boiler keeps cutting out

I had a Worcester 24i combi boiler installed about 18 months ago, and it's been fine since then. Obviously I've not been using the radiators for the last few months, but I've been regularly using hot water with no problem at all.
However, this week, I started up my central heating, and noticed that one of my radiators was cooler at the top than bottom, so just bled it a bit to get the heat even. Thought no more of it. Then a couple of days later, I start up the heating, and nothing. It doesn't light, and I can't get hot water either when I turn on my taps. At this point I also noticed that the water pressure level had dropped right off, so I added more water in to get pressure back up to 1 bar.
Then realised that the reason I couldn't get it to light was because the cut-off switch had popped up, so pressed that down, and then everything seemed okay.
But now whenever I try to start up the heating, within a couple of minutes, the pump's making quite a noise, and then the cut-out switch trips again. However, it does appear that I can get hot water out of the taps with no problem.
Am I right in thinking that the pump is only used for the radiator side of the system, and that's why I don't have any problem with the taps? And would the fact that the pressure dropped right off be the reason why all this has happened. I have now checked all my radiators, and as far as I am aware, I don't have any more air in the system.
Is this the kind of problem I can sort out myself, or is it likely that I'll need to get an engineer in? Thanks for any help that anyone can give me...
Gareth
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Your boiler should be due a service (every 12 months) anyway,so have a engineer sort out the problem at the same time.
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You don't say what sort of noise
Is it air in the system or a rattling noise
If it's a rattling noise, it could be that the impeller has broken off the shaft in which case it will make rattling noises and not circulate the water through the central heating system. It also means that as the water isn't circulating, the water in the heat exchanger will overheat and trip the overheat stat
How simple a task it is will depend on whether you have isolating valves each side of the pump. If you don't you'll probably have to drain the system, otherwise you can isolate it and remove the body of the pump from the head -normally by undoing the four (6mm?) Allen screws which hold the two pieces together.
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geoff

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