Hello, this feels like a dumb question but:

does the hexagonal top bolt as shown in the picture below undo?

Thanks! If so, what's the best way to undo it? I really had a struggle a while ago.

I'm guessing this is a dumb question. I've never worked on this bit of any bike so I would appreciate any tips on how to undo this. Honestly when I tried a while back it seemed to be an integral part of the peice below it and then the bike stopped working anyway. Now I am hopeful about getting it going again I need to sort this to get through the MOT.

Yes this bolt has to be removed to change the bearings.
It is screwed inside the tube.
The threads are certainly rusted. You can try to heat up the tube with a torch, and when it's warm, give a good blow with a hammer on the screw. This may free up the threads.

Aha, that must be it. Thanks for taking the trouble! Neal