I have this same problem and I only have 1 key. On mine the key will open the doors and allow you to get in no problem but when you try to start it the light in the middle of the dash will flash rapidly.
If this is the same problem as yours then it is £38 inc VAT to get the key re-doded, these are prices from my dealer and I found they very. However the problem is probably going to be the transponder in the key, a replacement will be £92 inc VAT so you may need to spend £130 all in.
My advice would be to just leave it for now while you're only using it as a spare then get a new key if the time ever comes when the other key starts to give you the same problems.