https://i.ebayimg.com/images/g/pHYAAOSweEFbDw0h/s-l1600.jpgI have found this and it uses the rixen and caul twist fittings that bodgeit suggested. I don't know if they are fixing it to the rail as well. If they are the rail doesn't seem that long and if I did something similar it wouldn't be too annoying when carrying. Thank you bodgeit for showing me an example of what they look like. The 'pannier system' makes a lot more sense now.
I think the hook and rail type panniers are the best ones, but a lot of cheaper panniers do just have a board on the back piece which the clips must just be screwed into. I will have to decide whether I make a fancy pannier system or use the flexi chopping board to make cheaper ones. They will never be carrying vast weights, but I want to make sure it will stay on and that it is comfy enough to carry.
Lizzy, I had thought of the double panniers as I have a similar pattern from pennineoutdoor, but I am looking for something that can be carried around when I am not on my bike and just looks like an ordinary bag. It does look a lovely book, and some of the bag patterns are quite tempting.