Worcester Bosch 240 rsf demand ignite not working?

Hi I have a 240 RSF that has suffered previously from a leaking PRV, so I am familiar with represurising, bleeding pump and know all about the reset button. However the on demand ignition appears to have stopped working resulting in no heating or hot water. The boiler clicks to ignite with demand on either heating or hot water but fails to ignite. Can anyone offer any help please?