Thanks for all the advice guys.

Had this problem and it was the battery. Put it on a CTEK 5 charger (a brilliant bit of kit) and worked fine again.

I am keeping the old vent and plan to do the repair you described when time permits (another one of those jobs!?). That way if I have another problem with the vent I won't shell out the £218 for the genuine part rather replace with the repaired one.

Thank-you again.

Thank-you very much for the very helpful post and photos. It gave me the courage to replace the vent myself. I must admit I decided to replace the vent with a new one not wanting to take the chance of having removed the old one, carried out the repair described and then replacing only to find I need...