Have you tried getting someone there to call you to get the number they use ? If so, does it give you the 0870 number or a "number withheld" message ?
I would assume then that all the UNI numbers are provided by NTL, in which case if you can get a list of the "Geo" numbers and check it against the list at the
Ofcom website, you may be able to deduce the accomodation numbers from those in or around the sequence that are not used.
As it might be a longish job and take a few phone calls to check out those possible, I wish you Luck.
Don't forget to post your results if you find them.