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1st thing to try before cutting the plastic grill, is to use some long nosed pliers to grasp the cable behind the handle in the footwell and give it a tug, as often the cable just stretches and there's no enough travel in the stock handle to take up the slack and also pull the catch
if this works you can then adjust the tension at the bonnet catch
if this is no good then yes you'll have to cut your way in via the grill so you can flick the catch with a screwdriver
finding another good used grill should be straightforward and I'd expect to pay about £10 at most from a breakers yard
try these for spares: