Sedona Immobiliser Problem

[Mr-H]

I spent nearly 2 hours in a supermarket car park today because my car (55 plate 2.9 CRDI Sedona) would not start. The AA man, who turned up after an hour, did not seem to know what was wrong. He had put a diagnostic tester on, but no results/error messages came up. He sort of said therefore that nothing was wrong; but the car still didn't start. He eventually rang his technical dept and 1 of his colleagues (it turns out also a Sedona driver) suggested that the immobilizer transponder might be causing a problem. It turned out that the 'immo' light on the dash must be 'on' before the engine will start, and mine has an intermittent delay. Without the light, the car will not start. Is this problem likely to be more with the transponder in the key, or in the ignition switch please? I have never heard of this before. I do have another key and will use that - if I get no problem with that then I have the answer, but I would appreciate and AB thoughts please.

[TWR]

Change the Battery within the key & change it Quickly.