Yes I think you're right @Kerry Beal & @Daniel D, it was a quite successful chocolatier who I went on a course with who advised the cold room, so I'm not sure how they make it work. Heating the moulds up with a heat gun is making a big difference! As is using wire shelves (thanks all)
However I'm still left with the issue of release moulds and it's driving me crazy. All I can think is that the moulds are just slightly too thick.
Can anyone recommend me a 100g bar/slab mould that you use successfully with no release marks?