Boiler cycling - 10 seconds on 10 seconds off?

Myson apollo 30si, CH only, boiler thermostat on low, no room stat, radiator stat's on 4 radiators - 2 without.

From cold the boiler comes on correctly and heats up to the correct temperature (in my opinion), this takes about ½ hour – 40 mins and all the radiators warm up satisfactoryly. the boiler then cuts off correctly and remains off for approximately ¼ hr, the boiler then turns back on but very quickly turns off again (5-10 seconds), after about 10 seconds the boiler re-ignites and cycles 5-10 seconds on 10 seconds off.

The flue passage is clear, I have replaced the low thermostat (when switched to high the boiler comes on and stays on) but the cycling seems to be even quicker now, pump set to 1 but 2 or 3 make no difference.

I have just had another look at it, I waited till it started cycling then opened all the radiators full and turned the pump to 3, it continued to cycle for 20 seconds or so and then I think the pump gave up. I closed the pump isolation valves to try and free the pump but now when I open them again they leak profusely, and then the end fell off my multimeter so now I'm going to bed!

Any ideas would be very gratefully received.

Rob.

P.S. i can easily repair the valves so i'm not bothered about that, do you think it could have been the pump all along?