Thanks Nick. Thanks also go to Tony Sefton & Lee who had a similar problem & pointed me in the right direction.
To cut a long story short, I had to find something to read the Immo system. Thanks to Duncan at Hickley Valtone we re-matched the keys to the system with AD100 & all was OK, (apart from I had to put the dash back together after testing)
Why the NATS light seemed to suggest the keys were OK I don't know. I also don't know if this will fix it permanently, but time will tell.