Hi Paul,
On the 19 you will need the diagnostic computer connected to re-phase the key with the car. You will therefore need to book it in at a dealer, and like I said it should cost less than £50. Over-riding the imobiliser can have an adverse effect on it's operation but you won't know until the dealer attempts to re-phase the key. Worst case is a new decoder and plip keys (about £180 in parts) and about an hours labour to fit. (We have just done this on a 19 cabriolet)
Hope this helps.