Do you need a smoke detector/sounder in a small plant room off a landing within an existing building which has undergone a change of use to a guest house with an existing pub on ground floor?

The plant room is on the 1st floor hallway and does have a 30minute Fire door and is locked shut.

Yes to a detector as the required Category of fire alarm (L2) in this type of premises includes rooms opening onto escape routes and rooms of high fire risk so it falls in under both those.

A sounder would be needed if the audibility levels in the room from the nearest existing sounder are not adequate.