Where can I buy a quantity of Roundup or similar to treat
approx. an acre of ground, possibly twice. I have heard
that there's an "agricultural" or "concentrated" sort that
can be bought and is more cost-effective compared to the
garden centre type diluted appropriately. Any information
will be gratefully received!

Look under "Agricultural Supplies" in your local Yellow Pages.

You will need a backpack sprayer or a weed wiper to apply it, though. If you
do not have access to either, it may be more cost effective to make friends
with a local farmer.

Howard Neil