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