Cracked it! And I reckon this fix will be easy for others to replicate, as it avoids the need to use brute force, or to fabricate a complicated tool. All you need is a tool that many DIY-inclined owners will already have in their tool kits, along with a short (20mm) piece of pipe or strong tubing to use as a spacer.
The tool? A bog-standard brake cylinder rewind tool, which anyone who has ever replaced their brake pads should already have. Mine cost a few quid, fifteen years ago. It's perfect for this job, as it enables the wretched spring to be compressed safely and easily. Who'da thunk it?!
The fix? Remove the plastic inspection cover on the underneath of the part of the mirror assembly that fixes to the door. (For simplicity, I'm going to refer to this as "the bottom section" of the mirror, and to the part of the assembly that contains the mirror itself as "the top section". Apologies to any nomenclature purists).
Hold the threaded nut end of the rewind tool tightly against the underneath of the socket in the bottom section (i.e. the socket into which the metal cylinder locates). Locate the two sections of the mirror together, taking care to ensure that they are correctly aligned. Pass the bolt of the rewind tool through the metal cylinder and spring, insert into the plastic socket in the top section of the mirror, and the secure the bolt into the nut. This is slightly fiddly, but once the thread has engaged, you can proceed to compress the spring from the underneath, by tightening the nut of the rewind tool. I found that, to provide sufficient clearance between the rewind tool and the plastic socket as the spring is compressed into the socket, I needed to insert a short (20mm) spacer between the metal cylinder and the top end of the bolt. I used a piece of nylon tubing from one of the kids' old construction sets (any port in a storm!) but anything that's wide enough to fit over the bolt (eg, a large nut, some washers, or a bit of copper tubing) would do the job equally well. The bolt of my rewind tool is 950mm long, which was
just long enough to reach through both sections of the mirror and engage with the nut.
Of course, once the spring is fully compressed, you need to rotate the metal cylinder a quarter-turn, to engage it with the bottom socket. After a bit of trial and error, I did this using the method that others have recommended (i.e. via a few taps with a hammer and screwdriver on the indentations on the top of the metal cylinder).
I'm pleased to have found another fix, to add to the ones previously suggested, but it really shouldn't be this difficult to fix what is clearly a common problem (and an expensive one, if you ask your dealer to fix it.)
I reckon this is the tool I have
https://www.amazon.co.uk/Silverline...60&hvtargid=pla-431462481321&psc=1&th=1&psc=1