Hi and welcome to the club.
Had the same problem a few weeks ago with my 07 - 987. Had to call out the AA to get my bonnet open.
So here we go, this is how I succeeded in opening the bonnet.
When attaching to the Red fuse point in your fuse panel make sure that you have a "GOOD" booster pack. Connect the Pos to the Fuse point and the Neg to a good earth point on the car. This really is trial and error. Door hinge works well for the Neg connection. Now here is where the patience comes in. It took us a good 5 mins with booster pack connected just to get my key out of the ignition. I think the battery will need a little bit of a charge in it to get going. The bonnet still wouldn't open off the catch in the car.... We had to leave it a little while longer attached to the battery pack before the key could open the bonnet. The secret to all this IMO is having that good connection for starters.
Once in......
Disconnect the battery Neg first then the Pos terminal. Try charging it up although the battery may well and truly be goosed. If it does charge up. Replace the battery with the POS terminal first then the neg. Things may happen to your car, spoiler goes up, windows might go down or up depending on where they are positioned to start with. Your PASM light and other warning lights on the dash may come on and wont go off... Dont worry. Just start the car and give her a quick run up the street. I normally just drive her over 20 mph and it resets everything.
Not trying to insult your intelligence but I dont know your level of fixing things.... Just trying to help mate. Hope this makes sense however in my defence I have had a few glasses of the red stuff [
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BTW where are you in the region? I may be able to get a hold of a battery booster pack tomorrow if your struggling and need a hand.
Please feel free to PM me.
Andy