UKPassats

A few times now my 2001 Passat has started cutting out immediately after starting and saying "immob active" on the cluster. If I lock the car then unlock it a few times it seems fine again. This has happened three times in thje last two months and it is starting to worry me!! I've only got one key and can't really afford to get another one cut just to find out it's a fault with the car instead!
Any ideas guys?