ideal classic lxff & ff boiler?

my boiler pilot light lights but boiler will not fire up. eventually after 45 mins clicking system on and off it fires but there is a lot of clicking sounds and the boiler goes on and off a few times then settles down. it then seems fine for the day if you leave it on but if not it wont fire up or the same thing the following day. we have changed a number of parts in the last year as boiler has kept tripping out

D SIMM
January 2013