Try any local auction house that does fine art sales. Biddle and Webb, in Ladywood, Birmingham are a top class antiques and art auctioneers (0121 455 8042) .They should be able to help.They have a very good reputation.
Or try London: Christie's (South Kensington branch). They do free assessments and valuations.You don't have to attend: you can try sending a photo with as much description as you can (size of picture, whether it's in oil or other medium) Sotheby's in New Bond Street do the same. See if you can decipher the signature (it doesn't matter if you can give only bits or you can read different versions of what it might be, just give them what you can)