Hi Speysider.
Loads of info. on here about tyres. Worth reading it before you purchase new ones. Use Search and type in "tyres".
Regarding the spare, it's probably worth changing it for the sake of 40 quid but we were obliged to use our "new" 1998 spare a few years ago while in Barcelona. Although I constantly checked it for the rest of the journey, it performed as new until well after we got home. We did have it checked out however and were told by our local tyre dealer that there was nothing wrong with it, on a few occasions previous to having to use it.
Randa