More good news, I'm down to 13 stone 10.
And I filled up on petrol at Tesco. No queues, no shortage.
With three keyboards fixed, I couldn't stop now. Keyboard four has both ctrl keys and both alt keys not working .. and a few more. I can't use a keyboard that doesn't have a working ctrl key.
I opened up the plastic case, noticing that one of the screws was missing. That won't be a problem, I'll replace it with a Phillips head screw. I used the chisel to remove the rivets, that is now very quick and easy.
I dismantled the inside, removing the key caps for cleaning. I've found another trick; to separate the two parts of the key cap, you squeeze the outer (with the letter embossed), and the inner comes out easily. But you have to squeeze north-south. I left the key caps to soak, and cleaned the plastic sheets. When they were dry, I tested the one with the narrow ribbon cable. And, as I had thought, one of the traces doesn't make contact with the connector.
My first try at fixing this, is to use silver paste at the connector. It'll have to dry before I can see if that works. I left it overnight.
... much later ...
So then I tested the plastic sheet with the wide ribbon cable, and the first and last traces didn't work at all. So I've used silver paste to fix this, but it'll have to dry before I can test it. Meanwhile, I've spread the keycaps out to dry, and I've drilled the black plastic keyholder. The drilling is a lot easier with the Dremel at 1/16 inch, followed by a 3/32 drill, used in a power screwdriver.
I've also ordered 25 of the screws that hold together the plastic shell - I really only need four, but they come in 25s.
And I've ordered 100 more nuts and bolts, and ten 1/16 drills, and ten 3/32 drills, all for future use.
And a chuck to fit on an electric screwdriver; because then I can use any drill bit.