Thanks everyone. I have bought the 125/70/18 with 5 X 100 bolt pattern, which came from an 01 A3.
I wanted a spare that can be fitted with minimum faff, and a spare you have to (hope will) inflate doesn't qualify.
The +6% on circumference might, possibly, theoretically, cause warnings if fitted to the front axle, but, these would be a minor problem at worst. If you have punctured, you just need to get out of danger. Anyway, you can always turn ESP Off.
The offset (+30mm ish)/clearance is a non issue, with the space saver half the width of the alloy!
Ohh ... it fits in the boot too!
Now I need a false floor, is there an OEM item, or is it DIY?
Mac.