Thanks WW, but its more fundamental:
A close inspection has revealed a burnt out connection. The componenet that sits just above the blue input terminal block - I think it’s a thyristor, has blown - it’s the central connection that has gone. Has anyone else had this problem and might there be an underlying problem? I am hoping that if I search on the printed number I can find a replacement.
That’s the thing- I tested it for 5v a while back. I just wired one with a higher output into my old Landrover to replace the electro mechanical voltage stabiliser. I suppose I need to find something that converts 15v to 5v.