This seems like it is going to be a ball ache
I spoke to BTOR infrastructure. Was a slightly surreal conversation as it turns out their office is actually on my road(!). I walk past it maybe 15 times a week.
Yes, the service arrives underground into the cavity between the two houses. Despite apparently there being a service hatch somewhere nearish the property according to the lady I spoke to, I'll be damned if I can see it (I wonder if it is in a flowerbed).
I have two options... a basic "overhead" engineering fee, which though the service is underground, if it could be within the realms of moving the cable a little within the houses could be done in a short space of time. £127 inc VAT.
Any more needs a survey at £300, plus the works.
It looks like the basic engineering fee route is a non-starter. The cable coming out of the ground into the cavity between the two houses is quite tight, and I probably couldn't move it closer to where it needs to be without having to make holes in both my neighbour and my walls and having their internal wiring re-done as well if there isn't enough slack. (to give us both access to the new location).
The survey route is basically spending £300 to find out how much more it is going to cost.
Unless I'm missing something in the way of a route open to me; in a pragmatic mood I'm thinking better to re-make my line connections with jelly crimps to make it secure and likely to survive, cover the access panel as desired and if the worst comes to the worst,
and the line degrades
and my neighbours won't allow access
then I'll be into a survey fee situation, in the meantime I'm saving the cost.
I'd love to leave access, but I can't see how to leave anything that could be considered useful access given the room layout. At the end of the day, someone needs to be able to get access, get in tools and operate them, and be able to see what they're doing, for it to actually be useful. I'll have a word with the builders when they get here tomorrow to see if they can see a way around.