Great joy!!!!
SHE STITCHES!!!!!!!!! I went back to basics, and removed the needle plate and the feed dogs.
There is a fork-type part which fits below the needle plate, and which I had thought looked as though 'something' ought to slot into the gap in its 'fork'.
I had put the parts back together as I had first found them, following photographs I had taken on disassembly and originally, according to my photos, the gap in this fork space was clear.
So just now I removed the fork, and slotted the bobbin assembly 'finger' into the fork, and then screwed the fork securely into place.
I have put the feed dogs back and the needle plate, and the bobbin case and bobbin, and threaded her up, and .... she sews!!!
It makes sense now as the bobbin stays still while the hook turns around it. Without the 'finger' set into the fork space, the bobbin would not have remained stationary. So to greater minds than mine it would have been instantly obvious. Never mind, I worked it out in the end
B15erk and arrow - thank you for your helpful comments
I am very glad I do not have to faff around with those screws!
And yes, one of my machines which I acquired for next to nothing was sold as not working... turned out the problem was just a short piece of thread between the bobbin and the race, which took a couple of minutes to sort out, and now that machine sews beautifully
The next step is to mount her in my treadle base... watch this space!
There is also some more painting to be done. I mixed up what looked a perfect match using 3 different pots of enamel paint. Then to my dismay the paint dried several shades darker than it was when it was wet
That was on the top of the machine, where much paint was missing; at least the rust is gone now and the bare metal is covered, even though the colour doesn't quite match.
The same happened with the handwheel too - it is darker in the places where the new paint was applied. But at least the handwheel is no longer covered in rust, lol!
At some point in the future I shall do some paint tests. I have the approximate recipe, it just needs to be slightly lighter. I intend to at some stage mix up a pot full, and then I will have it on hand for the 800 series Berninas that have found me over the years
In the meantime the 840 looks less battle-scarred than she did when I got her, and she is sewing again, and that is what counts most I suppose
I will post some pics later