I would never cut them with a Stanley knife, much too old and wise .
however you had mentioned cutting
I was simply saying carry on you`ll get there in the end hack saw better

heat or to be more precise, blowtorch is best, heatgun may take a long time,
if the bushes look pretty shot, squirt wd40 and some stilsons or water pump plier action will get it moving,
a good belt every now and again too ,,as you see fit