Hi Dave,
Have you insulated the walls and sealed the floor ? they will be a source of damp ingress.
Like you I 'created' my 'man cave' otherwise known as my workshop in a brick built garage, (didn't have any choice really) and so far (touch wood) I have not had any damp issues, my tools and metals seem to keep rust free, so I thought I'd share how I insulated it.
I lined the walles with a thermal wrap **LINK**
I literally hung it like wall paper from floor to roof, dab fixed with a cheap verion of no nails adhesive, then I applied a layer of celox (like you have used) I used the 15mm version, then a 6 mm ply covering to finish off.
For the roof, I used good old fasioned loft insulation, again with a ply cover to finish, then I have painted the floor with a sealing floor paint.
For the winter months I have a 2kW panel heater which comes on for 5 minutes every hour (on a time clock) to keep the chill off and humidity low, and so far it has worked a treat.
The only difference to you is the loft type insulation which may allow more air flow than sheets of Celotex in the roof space.