Alan -

Yes, I normally ferment with the fruit or whatever for the first week
or so, which is normally when fermentation starts to subside a bit.

Potassium sorbate is normally used to prevent renewed fermentation in
wines that have some residual sugar. The recommended approach is to
ferment your wine out to dry, let it clear and rack off the dead yeast.
Then you add the appropriate amount of pot. sorbate and pot.
meta-bisulfite (more about this later), sweeten to taste and bottle.
Sorbate does not kill yeast ( a popular misconception) nor does it stop
an active fermentation. But it does prevent yeast from reproducing.
So if a few stray yeast cells manage to find their way into a bottle
(quite possible, as yeast is virtually everywhere) they will not manage
to reproduce and create enough yeast cells to cause significant
fermentation in the bottle.

Not sure what you mean by re-activation. By the time the wine has
cleared and you are ready to bottle, the original yeast are long gone.
You may get a few stray cells, but that is what the sorbate is for.

It's important to have adequate sulfite levels when you use sorbate,
since sorbate is effective against yeast, but not against malo-lactic
bacteria, which actually digest sorbate (or near enough) and apparently
produce some really unpleasant by-products. These bacteria are very
sensitive to sulfites, however, so normal sulfite levels should be
enough to prevent ML problems. You can test for free SO2 levels and
adjust as needed. Or you can just add 1 Campden table per gallon when
you add the sorbate, and you'll probably be fine.

Container terminology must be a little different in UK. In the US, a
glass container with a volume of a gallon or so is usually referred to
as a "jug"; containers in the range of 3 to 6 gallons or so are usually
called "carboys", and the term "demi-john" is reserved for the really
big glass containers, 12 gallons or more (45 to 50 liters) that usually
have some sort of wicker or plastic covering over most of the bottom
and sides. At least, that's been my experience.

Doug