[peterh]

Thank you Dave for your reply - and especially the picture and link to Rimmers part are much appreciated.
Sadly, there's still a 'but', as follows:

When I look through the hole in the slam panel from the top I can see the spring, which is both in place and working - in that it's attached to the safety catch and keeps that in tension and pulls the safety catch back to vertical when I push it backwards.
So far so good.
The problem is that the safety catch opening lever (which should poke through the bonnet once the main bonnet catch is released) doesn't!
Now I've taken the grille off, I can hold that little operating handle and swivel it up to the horizontal with no resistance, and then as soon as I lift it a bit further, it pushes the safety catch back and releases it perfectly in the way that it's meant to do.
However, there is nothing keeping that lifting handle in place, so as soon as I let go, it doesn't stay horizontal (like it used to do) but now just swivels back down under its own weight to a position below the horizontal, (which will put the lifting end behind the grille and difficult to reach without another tool like a hooked bit of wire) once I put the grille back in place.
So 'something' must normally stop the safety catch opening handle falling too far and keep it horizontal once the main bonnet opener has been pulled...which is no longer functioning, as the handle falls too far down (to the hidden position it normally adopts when the bonnet is completely shut). It's as if whatever used to keep the fulcrums of the safety catch and opening handle swiveling together is no longer there...whatever it was - another spring perhaps - or perhaps a bit has broken off the back end of that lifting handle (which can't be seen through the hole in the slam panel of course!) which normally stops it falling too far?
Any further ideas or suggested sources of advice much appreciated of course, and many thanks again.