As usual, you never stop learning things about your car, more specifically about the horrors and short-cuts executed by the PO when he rebuilt it thirty years ago.
The whole rear-end had been replaced (I have the pictures) but making sure the petrol tank would fit inside it only occurred at the end of the process, and the boot is essentially 1/4" too short ; the tank was only ever held in by two bolts, not three, as the holes didn't line up, and the boot paint was sadly abraded both fore and aft as getting the tank in and out required a fair bit of bashing and scraping.
Before

After

I have managed to grappille 5mm here and there (from the verb grappiller, which is to go through the vines after the vendanges and get the grapes that have been missed, I thought this was suitable for the refined polyglots on the Forum) on the tank edge, whilst remaining outside the rolled weld,

and it can now be got into place without force, and maybe even without scraping if/when I re-do the paint.
Careful ovalising of the big holes that take the fat rubbers means that I can now get all three bolts in, although sadly probably only two of the spacer sleeves - but it's a vast improvement.
Question 1 : what else should I look out for when doing all this ?
I was able to correct the alignment of the filler-collar on the car -
Before


After

using a pair of suitable sockets and a bit of 1/2 bar (doesn't have to be this EN32B from the Mini gear-selector rod, tho' it is lovely and stiff),

these make a good device for tweaking the angle of the top pipe without damaging anything, and I include the pictures in case this may inspire someone else ; this ought to make re-fitting the rubber linking sleeve much easier than it used to be.

Question 2 : do these parts look right ? Is there a more elegant way of blocking off the old aperture now that the pump is an external one ?

Question 3 : the rather nasty (unused and still new) gauze filter that I bought a decade ago, seen here, from that firm near Gatwick, doesn't suit the diameter of the messed-around-with pick-up pipe anyway, and it occurs to me that there must be a plethora of better end-of-pipe petrol filters on the market that would fit into the tank, and perhaps remain accessible via the sump extension ?
Any advice about what fits / can be made to fit would be much appreciated.















