So I've successful played this gem from my teenhood with only the black screen bug in Jack's bar causing me an issue until this final part of the game.

Having worked through to the end scenes I got completely stimied by the de-electrified fence, reluctantly I resorted to a walkthrough for a clue as to how to proceed. All walkthroughs say "use sword" but I no longer have the sword, I used it at the monastry/totemiser and it wasn't returned and wasn't reteivable. Have I missed picking up another sword or is this a weird bug?

There was a bug fix made to ScummVM for version 2.2.0 a year ago. It was part of a set of bug fixes for the player not being awarded points for certain puzzles. One of the unexpected consequences of that was that the game gave you your sword back when entering the monastery.

But even without that bug fix, I never had any problem finishing the game because when I needed the sword at the end it was back in my inventory again.

For details, see the following commits:

https://github.com/scummvm/scummvm/commit/496484b94bd5ad57f7a745b6246c912d7cc719e5
https://github.com/scummvm/scummvm/commit/022a65ee528538dd0f8937baa896ba24c7c0c377
https://github.com/scummvm/scummvm/commit/02a1552efea49bc9754eff0ae72a2dd8cdf21bb1

Belated but huge thanks for this.

I downloaded the latest edition of ScummVM and installed straight into the scumm folder in the GoG games folders. Worked a treat AND has given the game graphics a boost too. Now all I have to d o is redo the game from a pre-EndGame save. :)