The gasket will be pushed outwards. Once the mechanism sits correctly in the casing you then need to gently re-seat the base, using the charger as a "screwdriver" in reverse of disassembly - but, again, be careful as correct positioning is essential.

At this point, the little metal ring is placed over the screwdriver neck and clips into position. Reassembly is now complete.

Is there scope for editing / adding to other people's guides on here? If not I'll try to do a separate reassembly guide when I have to take it apart again after I get a new battery.

It will then sit incorrectly on the neck and, if you just jam the toothbrush back into its housing on reassembly, it will then be on the wrong side of the gasket and I'm sure wouldn't seal properly.

To reassemble, remove the metal ring from the toothbrush mechanism; don't force it, you will likely need to gently squeeze the sides to get it over the ridges in the toothbrush neck.

Then thoroughly clean all the built-up calc off all the seals to ensure it doesn't leak after reassembly and fail. With fingernails, scrape the calc off the mechanism neck (don't use cleaner here as the liquid will likely get onto the electronics). You can use gentle citrus cleaner and an old toothbrush on the metal ring, casing gasket, base gasket and thread.

(Just don't drop the little ring down the sink like I did and needed to disassemble the U bend!)

Gently push the mechanism/body into the housing, ensuring it sits correctly in the housing rails. If it gets stuck at the start it's not lined up correctly.

This guide is missing a really important aspect of disassembly - and, going by the photo of the toothbrush before disassembly, it appears as if the author had previously disassembled the toothbrush, fell for the same pitfall I have been struggling with and reassembled it incorrectly.

Before dissassembling your toothbrush, note that there is a metal ring which sits at the base of the toothbrush neck, above the rubber gasket for the neck. I'm not talking about the cosmetic chromed plastic above the LED that glows blue/green/red during use depending on pressure, I'm talking about the metal ring above it. If you compare your pre-disassembly toothbrush, you can see yours will have a ring which is missing from the author's image.

When you disassemble the toothbrush this ring will likely remain on the toothbrush mechanism, and force its way past the gasket.

It's probably got calc in the screw thread - mine did. I just had to press the toothbrush and charging base together with considerably more than "mild pressure" - I would describe it as considerable but controlled force; don't use brute force, as you especially want to avoid the charging base and toothbrush slipping since that will damage both like when a screwdriver slips in a screw.

The photo in the linked Marlin screwdriver set is of four Phillips and one flathead driver, rather than the described pentalobes, torx and tri-point?