It's finally open! The cable had become disconnected from the latch mechanism. Also, someone had botched a crash repair, so I couldn't get between the radiator and the engine block. Luckily, the passenger headlamp had broken top mountings, so I managed to remove the lower part of the bumper and pull it forward, remove the headlight, and then remove the air intake scoop. I was then able to get my hand where the cable should go and, by deconstructing a bucket handle and making it into a pointy rod, locate it in the mechanism and pull the bonnet release open. Now to buy a new radiator, intercooler,l and condenser as they are all twisted and look at getting some replacement headlamps 😢 😭. Bloody bodgers!