I've been mulling over the various suggestions. The back-to-back BiPAC 8200 VDSL2 modems certainly look feasible, but I only found one supplier not listing it 'as out of stock', GameSeek asking £107.72 per unit. Amazon is also listing the ZyXEL P-871M which appears to offer the same feature, but is quoting £128.19. So with two units needed, that's a fair old financial hit!
Price aside, there is power available adjacent to the POTS master sockets, and there would be room for the ONT plus modem, plus power sockets and switchery, which could be boxed-in without being too obvious. However, if the other flat owners wanted to go the same way (admittedly, rather unlikely), there is nowhere near enough room, so for the time being I'm putting this one on the reserve list, but not ruling it out just yet.
The false wall has the disadvantage of taking width out of the stairwell where every centimetre counts during removals so I have to rule that out. At the moment, that seems to leave me searching for some innocuous-looking ducting...
Thanks again for your comments, one learns something every day!