Adding my own experiences to this topic:
On my 2007 Focus ST - the bonnet lock deteriorated very quickly, I was able to open it twice by first pushing down & using just the key, after that it would not open when I turned the key right. A long screw driver did not work for me - spent around 2 hrs trying, then went under the car. I damaged the mechanism when I finally got it off, it's not clear what had gone wrong but a new cable etc. works fine.
If all else fails, take some time to read all the posts in this thread - I'll not repeat but everything required is there. Get a long, thin, 30cm screwdriver - £5 even at expensive shops.
It took me about an hour under the car, and another 20 mins jacking up & placing axle stands etc - not as bad as I'd feared. You will need a 10mm spanner (preferably with an angle for the ring end), a good torch / light (I used an off-road bike light - this helped), small hands if possile and a bit of patience to reach and undo the two bolts / screws on the back of the bonnet catch. Take your time, you may not be able to see one of them until you've got it partially undone by touch. When you have them both removed, ideally you need to prise the catch towards the back of the car - do what you can from underneath the car, you may be able to poke a long screwdriver throught the grill if like myself you had the first stage of the bonnet unlocked. the fitting has a few ferrules that ensure it is sitting in the correct location - these disengage by moving the catch mechanism back about 5mm. Then just give the bonnet some force up, careful not to lift too high due to the alarm sensor cable. When the bonnet lifts, you should be able to then access the latch and operate it manually (with a flat head screwdriver) to seperate from the bonnet & sort out the cable etc.
Fitting the new kit:
I was able to retreive the old lock by a combination of knocking out & drilling out the metal 'rolls' that hold the lock mechanism together. 2mm bit, possibly a 2.5mm bit, I had to prise apart using some flat screw drivers after one of the rolls / rods was partially left in, this was possible as it had been weakened by the drilling.
Re-assembly of everything was straightforward, as earlier people have suggested it's worth removing the screws for the front grill before you close the bonnet for the first time - it may give you options should something go wrong.