Reprogramming one click key fob

I hope that there is a Landrover buff who might throw some light on my sons predicament on his series three Discovery (L reg)

He has just purchased this vehicle and wants to ‘install’ a one button key fob, which he has separately purchased via E-bay. We follow the instructions in order to get the ECU into ‘learn’ mode well within the allocated time of 8 seconds, and the vehicle dashboard light comes on continually. The instructions now tell us that you need to press and hold the key fob until that dashboard light flashes. We press the key fob and its internal light flashes once but for however long we press it the dashboard light will not flash, as required for the ECU to acknowledge the key fob.

Does anyone have any idea how the key fob can be tested. Thanks in anticipation

If it's an L reg disco It'll be a series 1 not a series 3. It may have a 300tdi engine in it, but it is still a series 1. Very important as obviously there are huge differences between a series 1 and a series 3.

Are you sure the new fob is the right type ? There were 418 and 433 Mhz single button fobs.The frequency is written on the front of the alarm ECU and they are interchangeable,so its possible its been changed.

He has a 433mhz ECU. How can we tell if the key fob is at the same frequency. Is it written on the fob, maybe inside?