Martin. I went through the mesh with a fine tooth comb. After a wild goose chase trying to join vertices of the inner extruded edges I rewatched 1.2 BlockOut lesson are realised the mesh is supposed to be separate!
However I did find a few edges marked sharp and clearing those and replacing with a bevel weight has allowed the bevel node to work in the sense that different values now produce different results.
However it doesn’t feel like a good result. It’s supposed to suggest wear but all it seems to do to my eyes is accentuate the low poly mesh and produce highly unrealistic ‘wear’ on sharp vertical lines on an object that looks smooth.

Upping the samples only highlights the issue

At least I got to the bottom of it but it’s taken a day to do so 🙁