You sound like the guy I needed to be speaking to at the time! A great answer - thank you.
Thanks for your kind offer but it's not necessary now thanks.
To cut a very long story short - this is what happened:
The car a Mk2 Punto was daughter's car (passed on from her brother).
Daughter was away at Uni so I allowed the son to use it as it was cheaper to run than his lairy Japanese machine.
He lost a key at work when somebody had to move the car. He only owned up to it a few days before his sister came home. I went mad at him and told him it was up to him to replace it.
An original key from Fiat was around £150 iirc.
So he bought a blank key and took it, along with the car to a friend of a friend who was a mobile key man. I believe he did take the body computer out, but I wasn't there so I can't be sure.
So both keys worked fine and we were all happy. Then I had to disconnect the battery for some reason - but I am always very careful and the ignition would certainly have been off.
When I came to re start the car it was clear that it was immobilised. In the handbook there is a routine involving opening the throttle a certain number of times, and doing that enabled the car to start. But doing that before every start was not going to be an option, and as I didn't have the pin code I bought it from Fiat. I did speak to the guy that did the job, but as these sort of guys know full well, once you admit to having disconnected the battery who is to say I hadn't caused it? "Never happened before mate "etc etc. As my son already felt terrible I didn't want to make the matter even worse by getting in to a row I would probably lose, so once I realised that the pin code was not being remembered by the body computer, after some advice on tis forum, I eneded up at a scrap yard buying a matching set of ecus, computer and all keys. It took me a day but I swapped everything over and from there on it worked a treat.
So there we are!
You sound like a top bloke who knows his stuff- so good luck with your business. I always think that honest guys do well - I hope so anyway.
Cheers.