Well that worked, Grinling - many thanks for your advice!
I tipped the whole 'flat' cask back into the brewing container, added another kilo of white sugar, and some baker's yeast (didn't have any of the real stuff), and off it went! After a week or so it slowed as usual, and following your tip for timing, I bottled it in two-litre bottles and primed them as usual. It has cleared a treat, is now much stronger, and has a real buzz!
Mrs Growster's dad had a Courage pub locally, and he was the first to put on 'Directors', when it became the stronger draught beer on tap. I used to prefer 'Fremlins Draught County', which was a beer to die for, but like most of the gooduns, it was stopped by Whitbread, so 'Directors' became the creme de la creme!
This pint I have in front is already toasting you!