how do i fix my sime 80c boiler?

I reacently repaired a small leak just below radiator valve in an upstairs bedroom. Since the repair the boiler keeps starting and stopping. It manages to flame up but then after approximately 10secs switches itself off again. Whilst boiler starts up there is slight tapping/knocking on pipes around repaired radiator.
Would anyone be good enough to shed some possible light on the problem.
Cheers.