Renault Laguna 2.0 8v 1995, immobilizer fault?

This car was running perfectly.
The car's battery was changed, but fired up again after, with no probs', it was then moved out of the garage and parked a short distance away. Approx 10 mins later I went to start it (after pressing the obvious key fob) and nothing, the roof receiver led light failed to flash (this model does not have a flashing central locking button), the starter motor spins the engine Ok.
Changed batteries in fob, but nothing. Have checked for spark at plugs and have one, the fuel pump can be heard energizing so have fuel pressure,(not confirmed) some times the engine will fire, but only briefly.
Does anyone know of a way round this? Or is the inevitable Main Dealer visit my only way to fix this? ..... any ideas would be greatly appreciated ! Cheers.