Sigma S30 - Immobilizer problem??

Hi Folks. I have a Sigma S30 fitted to my 1998 MX5 Mk2. I removed the Car battery as it was faulty and after a few weeks I fitted a new replacement. Result ?? The car is immobilised, it would appear that the only key fob I have with the car has lost it's coding with the main unit, and as I bought the car a couple of years ago second hand, I didn't get the PIN No with it ( although it has a small keypad)
I know I can recode the fob, but to do this I need the PIN No for the keypad .... so .. stalemate.... any help would be appreciated