In the future
if I wish to keep the wine even longer,say a couple of years or so,do
I have to add more sulphite than you are suggesting-would it be better
then to add much more sulphite now or add it in smaller doses every 6
months.I suppose commercial winemakers have to add a lot of sulphite
on bottling as they do not know the date of consumption.

No, the guidlines for sulfite addition are not dependant on wine age;
if you follow the standard charts based on pH you will be fine as long
as you have a good closure. +/- 0.2 pH is not quite accurate enough;
I would suggest you see if you can find a way to get to at least 0.1pH
if possible or error with the uncertainty added; ie if you measure 3.4
pH assume it could be 3.6 pH and add the amount of sulfite needed for
3.6 pH.

The other point you raise is that it would help to bottle the wine
(and keep it cool) to make it safe.I do not really understand why wine
bottled is safer than wine in a glass 1 gallon container,unless it is
the problem with inadequate closure.

That is what I meant, apolgies. You need a good closure and need to
minimize contact with air (oxygen really).

At the moment,the glass container
has a glass airlock (incidentally,we cannot obtain glass airlocks in
England anymore-only plastic which of course are porous to air.Is it
possible to obtain them still in the U.S.?) and I am leaving it like
that for a couple more months in case of some more fermentation;then I
will tightly bung the glass container with sufficient headroom to
allow for expansion as the weather warms up,using a rubber bung.

I haven't seen glass airlocks either, plastic is fine as long as you
trim off the excess sprue. Sometimes the mold lines are pretty
prominent, just shave them off and you will be fine. I have had wines
under plastic airlock for over 2 years with no issue.