The message
from Alexeix contains these words:

Back in August, I planted a couple of grape vines (one white, one red),
in containers, in a south-facing position.

They initially did quite well, but are now looking a bit sorry for
themselves.

They will - it's autumn.

What can I do to make sure they survive the winter and prosper?

Neglect them, but don't let them dry out completely.

I don't have the option to winter them indoors, so should I put up a
plastic cover, to stop the worst of the frost?

No.

Is it too late in the season to give them feed?

pedant

No.

/pedant

But it wouldn't do them any good.

I will post back with the grape varieties and details of the soil I
used.

Doesn't matter. For grape vines in containers, you need very big
containers, and very deep. A 40 gallon plastic drum might suffice.

Any advice will be much appreciated!

If you can, plant them in the ground and train them where you want them.
Then get or borrow a book on viticultu really, there's so much to
grapes that odd snippets of information aren't going to help a lot.

--
Rusty
horrid dot squeak snailything zetnet point co full-stop uk
http://www.users.zetnet.co.uk/hi-fi/