Thanks Norman, it is  impossible (for me) to get up and under to locate the ‘nut’ as such

What I did notice was that with all my pushing trying to get it to bite the thread on the screw was a bit damaged.  I found a longer bolt and it grabbed nicely and it is secure.  I suspect the thickness of the material was just too much for the screw to grip.

Anyway, minor issue but fixed so that’s good

Cheers

Martin