How can I turn off the immobiliser in an MG (P Reg)?

The key fob is broken, (long story!) so isnt working to unlock the car and turn off the alarm. The car can be unlocked manually, but the immobiliser refuses to turn off, so the car wont start. The hand book provides a code, but that doesn't work either. I want to get it to the MG garage so I can get a new key fob programmed, but I need to get the car started first!