One star is too much. Total rip-off. They did send someone the same day but at a cost of £150 plus VAT. He said the automatic air vent needed replacing and they would send an estimate. The estimate was £65 plus VAT for the part plus £225 plus VAT for fitting - total £348. The boiler was left switched on but with a notice saying "Danger do not use" and it has been working fine for the last 6 days. I googled the part and found it cost £13.20 including VAT so I refused their estimate and ordered the part myself. My usual plumber came today to fit it and said the part did not need replacing and nothing further needed to be done - so I now have a spare AAV and just paid the plumber for his petrol.