Firstly, thank you to those who have kept me company on this thread, particularly Silicon Wizard, it's a lonely business trying to solve something in your little electronics lab when nothing makes sense...
Anyway so I added 4.7uF MLCC to the output side... no change.... added 4.7uF MLCC to input side no change!! Curious!
I looked REALLY hard at the component selection but despite the Vf of my diode being 700mV.... and having doubts... I then decided I must be fundamentally on the right track and there must be a gremlin...
R31 and R34 form the feedback divider... (Now this is the top layer... so no imaginative silk screen mirroring going on but...) (See PCB Layout below)
Swapped the two components on the board and...
[Spoiler: The R34 label is next to R31 and vice versa, must have got mixed up in layout ]
OMG... two days (on and off )... of my life!