A few dry joints were found on the reflow-soldered PCBs.
A thorough visual inspectio, after assembling the board into the box, would probably have found the problems.
I was always taught and in turn, when a telecoms instructor, that visual inspection is he first thing you do when looking for faults. Power supplies are second!
In the case of Anglian transverters a visual inspection is also essential after the board is seam soldered into the case. It is quite easy to disturb parts if care is not taken during this part of the assembly.
Yes, both units are now working correctly!