Thanks for the reply.
How long an Alan key? I did try that, but the Alan key didn’t grip onto anything.
I’m trying to turn the wing nut from below and it’s won’t tighten fully.
I have a mixer tap in my downstairs toilet and the mixer taps are loose.
The fitting underneath looks like this (see pic).
I can’t see how to tighten the brass bolt up from the bottom without the flexi pipes getting twisted constantly.