Gort, i think you will find there called ghost numbers thats what i have heard them refered to in the past
I think you have three options
find someone in another area willing to connect a phone line for you in there house and then just divert the calls, you will have to pay line rental though wich i think is about £15 a month.
ask bt for the ghost number thing, but i was told its only for buisness users and costs around £500 to set up
or just use one of the forwarding agencies theres loads about do a search on google or somthing, as i say i think the going rates about £15 set up and then £3 a month plus call costs, by far the cheapest option