If anybody else needs these contact details:
Royal Mail Fax Number (to be honest, I’m not sure where this one goes): 020 7250 2030
Royal Mail Redirections Centre Fax Number: 01782 406 120 (however, faxes sent at 1pm on a Friday won’t get looked at “until Monday, possibly Tuesday and then will take 5 days to action”)
Royal Mail Redirections Telephone: 08457 740 740, option 2 2 4. However, extremely busy at 4.30pm on a Friday (20 minute hold times!) and reasonably busy at 2.40pm on a Friday (8 minute hold time).