Sorted it, managed to lock a pair of grips on the linkage from underneath, cheers.

i can fit my hand thru the grill and pop it open

You can jack the front of the car up and reach up into the engine bay and grab the cable going to the release. If you pull on it har enough then you can release the bonnet and open it as usual. If that doesnt work, cut some holes in the bonnet where the hinges are welded on and where the clip is welede on, if you really don't care about breakin the bonnet.

Can you open a Corsa bonnet without pulling the release lever in the car? If this poses some kind of security risk feel free to u2u me instead.

Also it doesn't really matter if the bonnet gets damaged in the process as long as nothing else around does.