Short of phoning them up and asking, you can't. The BT Osis file requires an address and name in order to give you a phone number but will not give you a name and address with only a phone number. The only other way is if you know someone that works in directory enquiries to tell you, but they are not actually allowed to do that!!
D'you have a name to go on as well, or is it just a phone number? I would try www.192.com - it's a fantastic site - especially for stalkers as you can pretty much get any information you want there!! Not that I'm a stalker - I was just saying... !!
Hey nigividitch that site is quite amazing. I have found a relative on there and would like to get in touch. All I know is she lives in blackpool somewhere, so could I find her telephone number and address if I buy credits?