Looks like the WiFi mesh was the right solution for the horrendous WiFi coverage in the new flat.

3 little mesh routers later, and I have excellent speeds everywhere on both floors, and, more importantly in the office where I edit / publish stuff.

@thelinuxEXP I'm amazed at how much more reliable my signal is after switching away from the ISP supplied router.

Now I'm looking around for a mesh setup with VoIP and VDSL support for my parents' place... that's a bit harder.