Part 2 of this operation was undertaken y'day.
I spent over an hour digging up the ply covering the width of 2 floor boards. Very quickly found one end of the floorboard. Never found the other end, I dug all the way across to the other side of the kitchen without finding it. And it was nailed down in the middle.
So we couldn't take it up and reinforce the joist with some wood.
Tried getting the next floorboard up in the other direction (my brother reckoned it was short and loose) but it disappeared under the cabinets. (So that was 2 floorboards abutting at the short end not screwed in at all.)
Doh.
After some poking around in the slot between the 2 short board ends with a stripping knife we decided there was prolly just enough joist under both ends so we screwed them both down. Then chopped lots of ply and screwed it all down. My brother decided to go round those boards with silicone to help stop creaking, which was a good idea but poorly executed because he decided to do it before we'd got the ply precisely templated to the hole so everything, including him, got covered in the stuff, which was amusing.
That appears to be the most irritating offenders not creaking now. Fingers crossed they stay that way.