The 0800 number was probably for the TPS (Telephone Preference Service), you can go online and register
here. All they do is add you to a list of do not call numbers for telemarketing companies, the companies can then reference this list and remove you from their lists. There's also the MPS (Mailing Preference Service) for junk mail
here.
Regarding getting your number changed, I believe it's possible. There was the high profile case of
the mother given a young nympho's number who was offered a number change. Should be able to call CS and get it sorted.
Hope that helps