With either HW and CH my boiler just clicks, the pump starts,the fan starts, but no ignition and this keeps repeating with no spark.
Also in summer position turning on a tap produces the same results
Their is gas available.

David Tinto, February 2006

If fan fires up every time it is not the air pressure switch,It is the ignition box fastened to the gas valve which is faulty,it is a common fault,make sure it is either blue or blue/turquoise in colour so you order correct one,they are about GBP95 including vat ,Robert corgi reg gas engineer

Robert, June 2008

try your air pressure switch, its job is to prove the fan is working and lets the gas valve open then ignition proving occurs if fault is not at air pressure switch, faulty pcb is most common fault

colin the gas fitter, April 2006