Sabre 29 HE running cold!!!?

Looked for answers here but unable to find any. Had the above boiler for the last 2 years and its been fine. Had my gas meter moved and since then the boiler runs cold after a few minutes. yested this on all taps and its the same. when it 1st happened the red light came on, now, the fan kicks in the burner lights the water gets hot for about 1 minute then the burner goes off and water runs cold. after about 3/4 mins the burner relights the water goes warm then the same process happens. i tested the CH also and its the same thing. Atfer the men moved my meter i asked my wife if it all went ok as i was at work and she said they had a problem to get it started at 1st and said it npossibly had an airlock, which cleared after a few mins. Could this be the fault and if so how do i clear the airlock.
Thanks for your time.

Jonny