I decided to try my spare key for the first time yesterday, it opened the doors and started the engine for a couple of seconds and then cut out, the key light was flashing which I presume is the immobiliser light. The main key still works perfectly fine.

 

Any ideas?

Your spare key may need resynchronising. That's been covered before.

I can only find how to program a key fob, my spare key doesn't have any buttons, its just the basic key. 

@Aidan84 If it's just a dumb key, it won't have an immobiliser RFID chip in it I think.

Even the basic non button key should have a chip in it, it's only the emergency keys that didn't then you had to use the emergency code to get around the immob

 

@nige8021 Mine came with 2x 2 button plips; so did all of work's pool cars. Also, we don't know that the OP's non-plip is a factory key, rather than an aftermarket and I doubt he will either.