are you talking free standing brush shredder or proper large chipper?
they are different beasts,
if you have been buying from b and q i guess its more of a shredder you want for stuff upto a couple of inches diameter max. look out for a camon c80 they are about £700 but you might get lucky and get a bargain, if you get it serviced once a year and learn how to keep its blades sharp it should do you for a very long time...