Given the previous unhelpfulness of your former tenants, I wouldn't be at all surprised if the guy has provided deliberately misleading information anyway.
Most National Express coach services are actually run by local coach firms who've simply signed up to have their coaches painted in National Express livery and to operate services as a sub-contractor. For example, the coach drivers (and office staff) around here in Suffolk who nominally work for National Express are actually employees of Galloway Travel Group, which is based near Stowmarket. (So, if your tenant had been one of their coach drivers it would be Galloway that you'd need to send the attachment of earnings form to, rather than National Express).
It would be interesting to know which local depot the guy works out of, as that might provide a clue as to who his real employer is.