Ok just to conclude on this topic I started, I have decided to get a full size spare and then manufacture some sort of support for the boot floor cover.
A full size spare in the size 185/60/R15 + jack and brace can be obtained for £115 from a place called wheelshop. Then you just need the retaining bolt (also available for £10-13).
I read that 10mm of ply / mdf cut to 800x500 should be enough to support the boot floor, alternatively a large piece of ply + automotive carpet to create a whole new boot floor cover.
In total, for less than £150 should have a full size spare.