We had this problem on my dad's ST, in the end we had to jack the front up, undo the 3 bolts for the lock and then get it open that way, a huge ball ache..

I read something about it's a safety thing so you only have to push the bonnet down too hard and the plastic tube snaps and jams the lock to prevent theft.

Now we have to use an archery arrow to push into the metal wire and turn it that way

Or you could just go to ford and buy one but we couldn't be bothered. There might be another way, but that was the only way we could find at the time!

Hope this helps somehow