Richard, thanks for the information.
I saw the grab handle on the Brit Car website, but I have bought a used, very well cleaned one from a breaker, Tom Peers. Leolito is correct about the price - the new one is around £24 including VAT, I have paid £5 plus postage.
Again - the fasteners, as leolito acknowledges, fit the carpet-type trim in the area around the back seats and boot area. The first fastener, that is pictured, is used where the Harmon Kardon CD unit is installed, on the passenger side in the boot area. There are four of them holding the outer carpeted trim panel around the CD unit.
The fasteners in pics 2 and 3 hold various bits of trim together, such as under the folding boot cover. There are about 4 or 5 on each side, right behind the rear seats. In my vehicle they are a couple of different colours, Brown and Beige, so I suspect somebody before me has had this same problem - lack of supply. I think that I have sourced about 8 or 10 at this stage, from different breakers.
The big fir-tree fasteners are light Grey and do fit the seat base trim panels. There are a couple of short [10/15mm] ones around the side of the seat panels and one big one, beside the transmission tunnel, about 20/25mm long, much bigger than any of the others and easy to find online.
To some extent, I am picking up some spare ones because some of the ones in my vehicle are a bit stripped of the fastener bits.