Quick Update - I failed!
Well, I did manage to get the grille off without any damage to the bumper. Broke one of the tabs on the grille itself, but it's still perfectly usable.
Once behind the grille, I could not see the catch or anything to pull on to release the bonnet, so I quickly drove down to the garage I use before they shut (this was Saturday).
The guy had a play with it for 10 minutes and said, he couldn't see how it's supposed to be done from the grille, however, he had done one of these before, but from underneath, taking off the engine cover etc and with the car on the ramp.
So, I dropped the car off to him today, and got it back all done. Only needed the short cable (I bought both as I didn't want to be off the road whilst it was being ordered as they are non-stock items). Charged me half an hours labour - not bad I think.
Still would have been cheaper if I could have done it myself
PS, I did take three pictures in anticipation of doing this myself, but as I only got as far as taking the grille off, I don't think it's worth posting those pics (with the words "pull grille off") :lol