oil fired boiler problem?

Myson Velaire
boiler works all summer on hot water system but as soon as you turn on the cental heating system the following occurs
the boiler will run to temp and shut down normally 30 secs after shutting down the lock-out light comes on - the boiler will not re-start while the light is on. You cannot press the re-start button to re-start the boiler until the temp in the boiler has gone to almost cold. any ideas please help!!!!!