sabre 29HE system red light comes on every 20 minutes?

I have been having endless trouble with this boiler but had reached the stage where the red light only came on occasionally. I then turned the boiler off for a 10 day holiday since when it has gone to red light every 20 minutes. We have replaced the NTC sensor but it has not helped. Previously have also cleaned venturi etc. - is it possible these need cleaning again because the boiler was turned off? Any other ideas?? thank you!