If you ex-hedge has largeish roots, then drilling into them and pouring in some expensive root killer from your local Garden Centre will work. Or do the same with old engine oil (cheaper!)
Alturativly you could dig the roots out, but that would take a fair bit of time, dependant on the size of it
Another line of inquiry is these people -
http://www.bbc.co.uk/dna/mbgardening
Helpful bunch of people, regarding - how to, whats this and all things garden based. The only downside, is that there is no proper search engine, Beeb won't allow it

But you'll find a third party one on allotment tales.
Hope this is of some help - Woolly