potterton housewarmer fault?

potterton housewarmer 45 back boiler starts normally from cold, But when it restarts the pilot is normal but the gas supply goes straight to main flow not soft starting , causing ignition of the burner supply, if i switch of and leave it for ten minutes when restarted its ok again, this can happen all day/night , but if left to restart normally after the first time it will always go straight to main gas flow as above.