A common fault in these boilers is the air pressure switch sticking. Somewhere near the top right corner, inside the firebox, there's a small diaphram connected to a microswitch. (Top left on LS series I think). This switch is made by Honeywell, and can either be replaced, about £20 on ebay, or taken out and worked a bit. It only has 35 millibar to switch with normally, so a bit of dust or tightness can stop it switching. If you take it out, then fairly gently blow and suck into the white tube, you should hear it click. Give it a 30 second blow/suck workout, and it should go on for a few months, or years if you're lucky. Incidentally, THIS is the valve that Ravenheat ask you to bang the case to operate. NOT the injectors!
Madrigaul, September 2008