Wheel size & Spare wheel advice

Our Trident ( 2000) came fitted with 17" wheels but no spare. Doing abit of measuring and investigating, it looks like a 17" spare will not fit into the spare carrier underneath the rear ( which might explain why there wasn't one),  but I wondered if anyone has tried this or how others overcome this? Any other options for carrying a 17" spare as I'm not keen on travelling far without one? 

As an secondary question, has anyone got any views on switching back to the original 15" wheels? Are there likely to have been any modifications that would also need reversing if I did this? 

Would be an expensive way of solving the the spare wheel issue!
Any advice gratefully received

Cheers

I may be completely wrong on this as certainly no expert on wheels and tyres but it was my understanding that when fitting bigger wheels you then fitted lower profile tyres. The overall diameter would then be the same or certainly similar, if not this can cause problems with your speedo being inaccurate and also clearance under the wheel arch especially when turning.  I am sure that like many things this is a very simplistic view and am sure many people have done many odd mods to wheels and tyres so could be completely wrong.