Potterton Netaheat 10 16 intermittant ignition?

Hi,
Boiler is about 15 yrs old. It has suddenly started to act very strangely i.e. Switch on = ignites pilot & burner; after 2 minutes burner dies to a small flame about third of normal height. Because of small flame, heat required is never reached and burner won't go out. Switch off: Switch back on after about 10 minutes = pilot ignites, burner doesn't.