Just a thought I assume the cable has broken in a position that cannot be accessed from inside the cabin and then pulled using a mole grip? When mine snapped it broke just behind the pull lever in the cabin and I was able to use the mole grips to pull on the cable and open the bonnet, If not see below to release the bonnet.
The metal cover mentioned below will still need to be removed anyway to attach the new cable on to the bonnet catch mechanism.Method:
Loosen the wheel bolts, jack up the car and support on axle stand, be sure to find a good strong part of the chassis for this.
Remove the wheel.
Locate the metal cover, (see yellow arrow on photo below), the cover is at the rear of the wheel arch, under the catch plate and behind the small body panel forward of the door and below the bonnet.
The cover is held in place with pop rivets, three I think, drill these out. There are no fixings between the catch plate and the bonnet so it should come away without too much effort, road grime and under seal may hold it in place.
Once the cover is removed you will be able to see the bottom part of the bonnet catch assembly, it should be obvious what to pull or push on to open bonnet.
Snapped cables can be repaired using a bicycle brake cable from Halfords or other bicycle shop.
Use the new inner cable only
Fix the inner cable at the pull lever in the cabin first
Then pass the inner cable through the existing outer cable on the car
Trim the inner cable to length at the bonnet catch assembly end and fix it to the bonnet catch assembly
Check action of cable and bonnet catch, adjust until happy. Method of adjustment should be obvious.
Replace the metal cover and pop rivet into place. Reapply under seal if required
Refit wheel and return car to the groundRemember to take your old cable with you when you buy a new one as the end of the cable at the pull lever end has a lug of metal used to fix it in the pull lever assembly and several different shaped lugs are available.
Although your car is a 300 and mine the older 260, I don’t think there is a different bonnet catch on yours to mine but If so, it could be worth asking Philippe to ask Tony Boulet at Extreme Limite.