for starters the prima doesn't normally have a permanent pilot, it only lights when there's a call for heat! so generally you wouldn't see a pilot unless the heating or water had just turned on!
if you have drained rads it could be that you had an air lock in the system and the boiler has overheated! on the bottom grey panel underneath there is a small hole at the front near the middle about the size of a 10pence piece, inside there is a overheat button, stick your finger in there and press really hard on the button and if its that it should pop in and when the boiler is turned on the boiler should light!
hope this helps you!
kev
November 2007