Astra engine managament light on?

This problem went away for a time but is now occuring nearly evey time the car is started first thing. The engine sounds like a tractor for a few minutees and then runs OK. The light stops flashing and goes steady and then goes off when the engine is warmed up. The local Vauxhall dealer suggested it was a dodgy valve. This would cost nearly £1000 to rectify. Any fresh ideas please? I really do not want to spend £1000 on it