"Ian White" wrote in message
...

I'm about to take over about a third of an acre of what's meant to be
wooded paddock, but is largely overrun at ground level by brambles. The
previous owner has managed to return some of it to grass, but has really
only, er, scratched the surface of the problem.

How can I finish the job, as permanently as possible?

Plough the whole field and start again, then weed out new growths and they
will usually give up after a season. This is also the best way to get back
to a grass paddock anyway.