I got there and without having to visit Tesco to buy some nail varnish remover.
It was some clearer thinking that was really required.
I put the wheel outside of the forks and clamped up one end of the spindle with a fork cap. Used a metal bar and a block of wood to hold the brake plate still and then with everything else held very steady, and only about one grunt on the breaker bar, the brake nut came loose quite easily.
Even better, my '68 Triumph TLS brake fitted straight in, as I'd anticipated after reading other posts on this forum. Now I just need to get the lugs on the TLS plate built up.
Thanks to all for your suggestions.
Andy