Hi Jonathan, it's unlikely that there are both metric and imperial sizes on the same unit. i bought a special spanner to tighten the nuts. It has one side open (like a horse shoe),
so that the spanner can be removed after tightening.
If memory serves me correctly, it was a size 11 spanner and it fitted all the nuts except one of them. The one that didn't fit was either a size 12 or size 10, I can't quite remember without going to have a look. It was a size different.
I really couldn't tell you the reason behind it, perhaps the pipe that serves the back wheels is a bigger diameter.
Great work on bending those copper pipes without kinking them!!