Should usually just be a matter of days.
Great, thanks for this info.
How long have the other flats had their numbers?
I wasn't clear. The flats are conversions, probably less than 10 years old, and AFAICT they have never had phone lines. But a few of them show up in the address search, and it's this (and the post code search) that suggest no availability. I'm hoping the reason they suggest ADSL2+ but not FTTC is because FTTC availability isn't added speculatively if a postal code / address block has never/not recently received any BT services.
Point accepted about regular cab capacity. Guess I'll just have to wait and see.
How new is the block of flats, and are there (inactive) phone sockets installed, perhaps by the builder?
No lines installed in the build. I think the original (pre-conversion) had a line or two, because there's old BT equipment on the wall outside (which I'm hoping they'll get rid of, it's exposed and in poor condition).