OK, so I trawled the forum and found the destructions how to recode a key....when I got the car, one key worked the other neither opened the doors, nor turned in the ignition. First step was to have my local techie shop fit a new battery, still no luck.
BUT, the instructions I read did not include the warning that it would decode all other keys, so now my existing key which DID work....doesnt..........and dont I know it...cos the FD is now bending my ear about 'Bodging', DAJMAJ, and all the rest of their vocabulary.

I realise that the bad key has prolly got a bad transponder, but why can I not re-program the good key.......no matter how many times I try it, it wont re-program

Help wanted......feel free to be as patronising as you like,,,,,,,,,I need to restore street cred with FD
Cheers all 2007 TDV8 VSE Java Black
1994 MX5 1.8s softtop customised VW Diamond Black

You need to code all your keys immediately one after the other

There is a nack to it too... Can take several attempts

Ya thanks guys, turned out the key I had the new battery put in needed adding with an Autologic box. Once that was done, the ignition recognised it and was able to switch the ignition on. This confirmed the blade was ok and the transponder was working. So, following the threads on here, I persisted, and tried several times to reprogram them, and finally, success........

To help others, on mine (a 2007), there is a definite audible click when you put the key near the ignition, (which I assume is the immobiliser releasing), and
when the key is coded correctly to the door unlocking there is a definite 'clunk' from the door mechanism,
also it may help to know that, if the key is trying to code and is not managing to link successfully, the windows go up and down and stay in an open position, to let you know it hasnt worked.

So, many thanks to all, including the original posters, oh and also a shout for the guys at ILS Preston, who coded the key for free and are sorting out my wheel swap and fitting a towbar next week.

Happy ranging 2007 TDV8 VSE Java Black
1994 MX5 1.8s softtop customised VW Diamond Black