Potterton boiler fan constantly on and intermittent ignition?

My venerable old Potterton netaheat 10/16 mk2 has started acting strangely. Whenever the boiler is set to be on the fan is going all the time. It never goes off unless the boiler is switched off from the timer or the front panel thermostat. At the same time this started to happen the boiler started to intermittently fail to ignite. If I notice that is hasn't lit then a light tap on the front of the boiler results in the normal click as the pilot ignites and then about 30 seconds later there is the normal loud clunk and the boiler ignites. If I'm lucky it'll light again on its own next time but some days I have to keep going back and tapping the boiler to get it going again.
Any ideas?

Tony Ronan
May 2007