It's taken a year but I've finally had a look at everything and the problem is fixed.
I could kick myself though as I ended up where I began.
Nothing I did seemed to make a difference.. Then, just as I was about to despair, I happened to notice the 2 locking pins (I think that's the correct term) were slightly different in length, so I began to fettle the pins once again and after the best part of an hour and often wondering whether I'd made the problem worse, it all finally looked right. The bonnet does not pop up as I drive along either...
