I'm wondering what the best options are, when my tyres approach the end of their life I want to uprate the wheels and tyres without upsetting the geometry of the car.

I have the standard 13" alloys, and was thinking of the obvious Abarth 14s, I believe you need some spacer type wheel nuts and a lower profile tyre, but I am wondering what other options I would have.

What would be the maximum overall diameter including tyre, width, offset etc? Could I get away with 195 or 205 or would it just be counter productive?

If you have a 14"x6" rim then a 195/45 tyre would be ideal, but personally I wouldn't go any bigger than that. I've seen Cinq's and Sei's with 15s, 16's and even 17's on them, but IMO it's not a good look. Tyre width is relative to rim width, for example a 14x5.5 would be better with a 185/50 tyre.

Sorry just getting used to this forum......My arbarth blew the other day so we bought a standard car to keep us on the road. Can I swap the arbarth alloys over to the standard car?

Yes, they should come with spacers so swap those over too.