What type of stainless, please?

[Zing]

Those are all highly specialised conditions. I thought we were discussing marine fittings at ambient temperatures. See this paper for example but there are many others:
Stainless Steel Material In Sea Water

304L, LDX 2101® and 316L can all tolerate somewhat more corrosive conditions in systems which do not contain obvious crevices, provided all welds are of good quality and well cleaned.
•........
• 2205 or lower alloyed grades are usually not sufficiently corrosion resistant.

That's because performance is similar across the various corrosive test conditions, nonetheless here's another extract from another data sheet, this time for salt and going down to ambient temperatures. The relative performance is clear to see.

Pitting resistance equivalence numbers are also far better (35 vs 24 for 316 and about 17 for 304). See here: What is A Pitting Resistance Equivalent Number (PREN)? - Unified Alloys

 

[vyv_cox]

Not so according to my information. Here is an extract from a 2205 data sheet:

View attachment 176491

Look at this page on my website Crevice corrosion
Both shafts shown are probably 2205, although the second one, written a long time ago, suggests 400 series. I now know that was a poor guess on my part. I have also seen Jeanneau rudder stocks, definitely in 2205, that suffered deep corrosion pitting.

 

[andyingreece]

I was planning on mounting my 'new' sheet winches (Lewmar 43 STs) using 'stainless' bolts, which are about £0.50 each.

Someone suggested that I should use 'marine' stainless (I guess 316). These would be around £12.00 each.

I'm open to persuasion: all thoughts welcome.

You wouldn't pay that for silicon bronze! I manage a chandlery in Essex & these in A4 would be around £2.00 each.