I will be in northern Italy until the evening of Sunday 7th September,
and I, a couple of friends and my car need to be back in southern
England by very early (say 6am) on Wednesday 10th - ideally back on
the 9th.

The obvious way to do this seems to be motorail.  But google and
askjeeves searches have failed to locate any information (apart from
one train from Milan to Belgium, which does not run on the right day).

I could go from Milan, Innsbruck, Triest, Venice - or any city within
that circle, and go to Calais, or anywhere within a couple of hours
drive of a channel coast port.

My question(s); is this possible, and if so, where can I get more
details and book?

French motorail do a service from Nice to Calais quite regularly. Go
to the Rail Europe website for more detail.

I will have to bring my car back from Milan in December - have you
found anything that goes to Calais from Italy?

Hi - No, I've found nothing out of Italy to anywhere - I'm flying/hiring instead.