Thanks for your reply but I seem to have confused matters with my use of the term "temporary".
Here is why.
Both my wife and I currently own Skodas, me a Superb and the wife a Fabia. Both came as standard with a glue kit with the option for £100 of a "spacesaver" or "temporary" spare wheel. I hate spacesavers with a passion, no use whatsoever if you need to fit one part way through a long holiday drive with a fully loaded car and no space to store the punctured wheel. So in both cars we specified the "temporary" spare which is a proper steel wheel with a proper tyre fitted (but not the same size as the alloys on the car). You are still limited to 50 mph but at least the tyre can carry a full load for an unrestricted distance and the full size alloy will fit into the wheel well without affecting the luggage.
So I suppose I should have asked can a full size spare fit into the boot without affect luggage capacity.
Before the Superb I had a Mk3 Leon and when I bought it there was no option for a factory spare and the price from the dealer was stupid, he wanted £200 for the spacesaver wheel, £230 for a spacesaver tyre and £60 for a tool kit/hold down bolt etc, told him to forget it. I went on MyTyres and bought their cheapest steel wheel with a 205 55 16 tyre fitted (£75 delivered) and bought a jack from Halfords. It all fitted into the spare wheel well but lifted the boot floor a couple of inches which was perfectly acceptable. Considering the boot of the A4 Avant is bigger than the Leon this compromise would be acceptable again if the wheel well is big enough to get a suitable "real" tyre into. I have calculated the diameter of the T125/70/R19 as approx 658mm which is actually 1mm less than the diameter of the 225 50 17 fitted to the A4 Avant but it may be close. Anyone tried it?
Thanks again.