Looking at photos of Mont St Clair it seems as if the harsh conditions are a bit similar to those in our steep garden in Greece, which also has a very thin-to-nonexistent topsoil. Yet we find that many attractive Mediterranean plants succeed in rooting well into the rocky substrate, particularly lentisks, phillyreas, Lavandula dentata (especially the cultivar 'Dusky Maiden'), a wide variety of cistuses and rosemaries. South African bulbs also do well, as do the standard ground covers like osteospermums. Many of the plants sold by Olivier Filippi from his nursery in Mèze just across the bay from you should flourish with you, even without added topsoil. Filippi's books are full of good advice about getting great results from conditions like your own. (In one part of our garden we did add a load of topsoil but honestly have not found that nearly as satisfactory as the natural state of the main part - but if you do want to make some use of added topsoil I can't think of anything better than the advice given to David Dickinson.)