This can be a pain in the rear! But on a ramp in the workshop, isn't too bad a job.

Assuming the key turns:
Firstly, looking through the grille there's a rubber boot on the lock mechanism. Using a lockout rod, or long pick, pop the rubber boot off then you should see a white plastic ring. Try pressing against this with a long screwdriver whilst you turn the latch via the key. Get a colleague to pull on the bonnet. If this fails method 2 is required.

Method 2: key doesn't turn or above fails.
(You can remove the bottom grill and the bottom of the main grill to gain better access).

Remove the under tray and look up. Undo the 2 nuts on the bonnet catch, (3x long 3/8" extensions worked) this will not undo the bonnet catch (it's spot welded)! But does allow it to flex a bit. Then undo the 2x 10mm bolts holding the latch to the front panel (easier if you have an air ratchet)

Looking from the front, there are 2 tags either side of the latch ring, press them in and push the mechanism in towards the engine. It may suddenly pop the bonnet a little. Hopefully moving the bottom of the latch away from the front panel.
Now, from underneath, prise the latch further away from the front panel with a lever, and it should move away from the white plastic which is the cable/key attachment.
This will allow the bonnet to be opened.

The reason for failure, is the little white cross adaptor that fits between key/latch. Pop this out (it's a tight fit in the latch end) either replace it, or turn it around so the worn end fits into the latch and the tight end fits into the cable end. (looking at the ends you'll see that 2 clips are worn).
Reassemble, easy peasy!