Vizo boiler fault. Burner cuts out?

A few days ago my 3 yr old combi started playing up. The burner was cutting out while the stat was still demanding heat. Fan and pump continued and after 30 secs maybe it lit up again. Consequently the rads were not getting upto full temp. There was also a relay clicking about every 5 secs.
As its on maintenance, I called out a man. He hadn't a clue. He muttered something about the maybe needing a CH sensor and has gone off to order one. Meantime the fault has gone away.
On the basis that it's likely to re-occur it would be nice to know what the fault might be.
If anyone has any USEFUL ideas please let me know. Suggestions like 'call the manufacturer' are not helpful