My ideal Turbo 2 12kw condensing boiler takes ages to fire up (it's 19 yrs old)?

When the room stat or hot water tank asks for heat, the boiler light comes on ok, the fan also runs but it takes about 40 Min's before the air pressure light comes on. Only then does the burner light come on and the burner ignite. Then everything works fine. Once it's been off for a while, it goes through the same annoying process!