Re-my earlier thread on this subject. Just in case anyone else has the 
same problem, I managed to release the bonnet without causing any 
damage. I had a close look at the release lever inside the car (head in 
footwell, feet in the back seat) and found the outer cable was slack, 
suggesting that the catch was jammed. The catch is quite easily 
accessible from under the car (both arms up behind  the radiator!) and 
by pulling down hard on the bonnet part of the mechanism it freed it up 
so the lever then worked.

Much quicker than taking it to the dealers and I learned something into 
the bargain.

Biggles