Sounds like the NDB might not need a formal commissioning, but I'm not the one to advise you on that, as I'm not with the UK CAA. I recommend that you have a chat with their navaid experts anyway, from the point of view of using it for holding pattern practice with your students.
As a personal observation, I'd have thought that any half-way decent flight simulator program on a computer would teach your students as much as they need to know about holding patterns more cheaply. It mightn't give a real feel for the stick and rudder side of things but it will teach scan and orientation.
Anyway, I still think that a chat with the CAA will be useful in formalising anything that needs formalising. They'll even be able to tell you whether or not you need anyone with specialist experience to perform the commissioning. If there's been no changes to the antenna or power system, I don't think that they'll require a flight check as part of the commissioning process.