OK I have had a bit of time in the workshop. I took off the front wheel and the brake plates. The pivot mounts could not be moved by hand so I too them off and cleaned 50 yrs of grot and rust off them and re-installed.
I re-read the manual which says they should be "moveable by hand but not loose enough to move with road shocks" hmmmmm...
They now move freely enough I think, see the video here:
https://www.youtube.com/watch?v=eA53QYAquWs
Let me know if that seems about right or not.
I took a test ride round the block, the brakes are now about as good as they will get, still not great though.
I also moved the brake arms round to give me as much effective pull as I can.
Thanks for the help.
Adam.