Clarke. Nice videos and, from what I can see, the main problem seems to be changing down from 3rd to 2nd gear.
I would recommend removing the inner timing cover and, depending on your strength or ingenuity, try going through the gears by moving the end of the camplate up or down. Hopefully, you will be able to feel the camplate indexing when each gear is selected.
If you have a similar issue when operating the gearbox in this way, it will show the problem is internal to the box rather than with the external quadrant or stop plate and stop pins. If internal to the box, it may mean removing the internals after taking off the clutch so that you can better see what is going on with everything assembled on the cover plate.