worcester 241 rsf - ignition fault?

boiler wont fire especially first thing in morning.

have found that when boiler does not fire if i remove red pipe attatched to steel going to aps and suck boiler will fire no problem, have fitted new aps no difference.

i then thought it may be a problem with the fan, have tested this with independent supply seems ok.

any other tests or possible remedies would be appreciated.

many thanks.