Thanks Casper. On the that helmet lining now. This proved to be one of the more difficult parts of the build so far.
The lining consists of a piece of cloth with a spiral of thread sewn through it. The idea is that you pull the threads until it forms a dome shape. This is somewhat tedious, and involves a lot of trial and error. My dome ended up slightly smaller than the helmet inside, but never mind......

Then there is a suede band. Unfortunately the instructions don't tell you what length to cut it too. I looked at the instructions, cut off what looked like the same amount of left-over (there is supposed to be some spare to cover some rivet heads) and tried that. Wrong !! It was far too short

It wasn't too difficult to fix. I just added the spare back in and carefully placed it at the front so it looked like it was supposed to be a separate piece (my story and I'm sticking to it !!)

However, that didn't leave any spare to cover the river heads. Those are to hold the neck shield in place. I didn't like the patched anyway, so I devised plan B. Glue the river heads in place with PVA first, then glue the hat band and lining in, and then glue the rivets from the outside.
The band is actually just glued in place. It mostly overlaps the edge of the helmet and is glued on the outside. That is covered by the neck armour. The front is glued to the inside of the helmet.
This is what the instructions suggest.

This is what I ended up with. I think I got away with it OK.

The whole thing was a real pain to do to be frank. I just need to add the neck armour and it's finished (for now anyway).