Poitterton Netaheat 10/16 fan on all the time?

My venerable old Potterton Netaheat 10/16 mk11 has suddenly started acting strangely. The fan seems to be on all the time even when the boiler isn't lit. Sometimes the boiler fails to light, this problem started at the same time. A light tap on the front is enough to cause the normal loud clunk and the pilot ignites and the boiler lights a few seconds later.
Any ideas?