If it wasn't for the external brackets I'd look into fiddling the internal gubbins to make the valve work at a different angle.
But as the brackets would have to be removed to fit the wheel well I can't see it passing inspection in modified form.
From memory the next vertical size up gave another 7 litres capacity from 48 to 55, which I was sqeezing in anyway on occasion. so wonder if it's worth the bother and just fit a cylinder..... Aaaw but I want my boot space
What size is the spare on the saloon? I'm assuming it's full-size like the Estate? Ok, I'm assuming the Estate is full size since I haven't really paid attention when I've opened the cover!
If so.. you should at least be able to get a 630x250 tank in there? 54L in toroidal form or 66L in four-hole full toroidal form..
Probably easier to fit a thicker tank in the saloon, too, given you're not trying to sit it 'in' a wheel well?
On the contrary... Easier to fit a thicker tank in the estate as you have space to raise the floor... In a saloon it has to angle and drop into the spare well which, TBH, is less than ideal
Oh... And IIRCC I worked out you could fit a 650x220(200?) with no modifications (other than removing the centre upright in the wheel well is there is one) and then, obviously, thicker if you wish, raising the floor to compensate.
I looked at many options, including cutting out the bottom of the wheel well to make more room, although this seemed too much work for the sake of an extra 15mm or so :-/