Thanks Barry!
As far as I was aware, the pillarbox/zoom settings under 16:9 are to tell the box what to do with 4:3 content and has no effect whatsoever on 16:9 content. I have tried this combination of settings on my box and it made no difference.
I set myself up an avisynth script to show gradually increasing borders on my screen, and have worked out that the humax is clipping: 24 left, 20 top, 14 right, 16 bottom. Anyone know why or who to report this to to get it fixed?
I realise that I am using a scart cable when this is an HD box, but I don’t see that this should make any difference.