try getting someone to push and pull the bonnet while you pull the catch it might just have jammed up a bit, if this works spray some wd-40 on it and keep opening and closing it to free it up then put some grease over the mechanism to keep it working smoothly. i've had this problem on many of my vauxhalls and every time it has worked
had the same problem on my astra. the cable had pulled through the cable lock. took me about an hour to get the thing open. had to force the grill off and work the release lever with a screwdriver.
If the cable is still attached to the bonnet and has just stretched as you say, try taking the cable off the handle and pull it with a pair of pliers (be able to pull further)
same happend to mine , go to scrap yard and look at the machanism . I found a lever off a jack that was lying about that was bent at a right angle . Poke this through the grill into the hole that holds the bonnet and then just turn the handle . less than 2 secs