Thanks to u102768 who gave me some more part numbers

well, I've ordered the fancy platinum brackets (part 4), and made a start at getting the old bits off.

All the screws I've found so far are M6 20mm. The ones at the front lip behind the bumper (not part of the engine tray) have a T30 head and feel like they go into a spire nut (spring nut). All the nuts are rusty and one snapped away and spins. I will be fitting new ones but giving them a spray with preservative paint first and using Copaslip on all threads. I will be using hex-headed stainless screws on reassembly so that a 10mm spanner will spin them off. The captive washer on the existing screws is bigger than C-form M6 washer or the standard flange, so I will look for a bigger stainless washer. Edit: I have ordered M6 25mm "penny" washers as I can't find M6 20mm like the old ones.

I have not been able to find a supplier of stainless spire nuts (spring nuts) but I would have thought they exist. Yellow zinc plate is really not suitable for a fitting exposed to road spray.

Part 11 in the diagram in retainer M6 x 17.5 C2C10348. I can't make out what it is but it might be a Rubber Well Nut, which I have got (with a brass threaded insert) but at a price of 80p I might order a couple to see.

I haven't yet got to see part 2, Mounting Bracket C2C3874 which costs £20.72 so I don't know if I will need a new one. I presume it will be a rusty steel strip with some holes in it.

It is my expectation that when finished I will be a to provide a fitting list of non-jag nuts and bolts that any fixings supplier will be able to provide in stainless, if there are other members contemplating the same job.