Thanks, I tried the Image Solver in PixInsight using both the catalogues you suggest - for the default limit magnitude (12) - and I got the same results. When I increased the limit magnitude to 16 then the image solves OK.
I suspect this might give a clue as to why PinPoint fails when launched via SGP Pro since Visual PinPoint does find a solution on this image at a limiting magnitude of 14!! I guess this may be a question for DC-3 Dreams perhaps - is there a difference in the way star limiting magnitude is treated in the astrometric engine when accessed via the API and the Visual PinPoint interface?