Thanks for the replies folks.
I had missed the screwed ring shown in the S1 parts list and now I have looked more closely I see it was used up to a certain point on the S2 as well, together with a fibre sealing washer (according to the SNGB parts lists), but the screwed ring is definitely not used on the one I found on my S3, where the sump casing comprises 3 parts permanently joined by what looks like silver soldering. There is a simple flange soldered onto the tank with no insert either as far as I can see.
The sump comprises a brass threaded flange (unlike the repro which is all steel), joined to a steel pipe section, then another flange, steel this time, which carries a 3/4"AF hex to tighten the sump with and within it the drain set screw. There are no removable parts on either sump shown in my previous picture.
The current tank seal (C26310), supplied by SNGB, is a flat rubber washer (they seem to vary from 2.5 to 3mm thick) and from the parts lists appears to have been introduced when the screwed ring and fibre washer were deleted. The S3 WManual specifies a torque of 25ftlb whereas I didn't find a figure for the earlier cars although I may have missed it.

S3 components
