Well there's a slow process available to you Ralphie, but if you have the time and are reasonably convinced that Northamptonshire is probable it may be worth it.

Put Butler P and the location Northamptonshire in the bt.com phonebook search. If you just put Northampton you will be limited to that town only. Unfortunately the county name throws up a lot of listings for Oxfordshire Cambs Leics etc that border it. A quick scan down the list gave me at least two in Daventry one in Northampton and one in Rushden and that was before I got to the listings with a secong initial for example there's also a P D Butler in Rushden.

As I said this is a slow process and assumes that he's not ex directory and the phone's in his name. Then you need to be most diplomatic in ringing people who aren't him to eliminate that number. When I've had to do soemthing similar most people are OK about you calling to track down someone with a similar name, just don't mention Double Glazing!