Worcester 24i?

Hi drained down the system to repair a couple of weeping radiators, refilled the system to 1.5 bar vented all the rads everything seemed normal this morning the shower ran warm and the hot water from the taps never got really red hot,the boiler is firing up for 10-20secs then going out then fires again and goes through the same sequence over and over again
the same sequence of events takes place when the central heating is switced on, before the burner goes out there is a click from a relay or something,eternally grateful if someone out ther could diagnose the possible cause.
Thanks Craig