Help! Immobiliser not working!

Really need advice on this, as about to sell my Fabia 1.4 Comfort. Unfortunately the remote locking function stopped working a couple of weeks ago (changed batteries in both key fobs/ been through series of actions recommended on this forum) and now the immobiliser/alarm isn't coming on. As a buyer was viewing the car tonight, I locked it and the red light was flashing by the door lock. When she practised for herself, it didn't come on... and then I tried umpteen times, but the door light & dash light are not flashing, indicating that the immobilser is not active. This has postponed the sale (to an otherwise really keen buyer) until I can get it sorted, as its a key insurance issue. Does it need to be reset, along with key fobs or is it something I can redo myself?

Would really appreciate advice on this.

Cheers!