Somewhere along the line the Bootcamp install on my Mac got very messed up, or alternatively, v5.0 of Apple’s Bootcamp drivers did something very weird with Apple Software update – either way, I’d upgraded the existing installation to Windows 10 and wanted the new Bootcamp 6 drivers and they weren’t showing up.
The solution was simple, but a bit tedious. I downloaded ~800mb file containing Bootcamp 5.1 and ran setup. It asked if I wanted to Repair missing files, which I did, and after a reboot was told I was running 5.1, and at that point running Apple Software Update finally started showing the v6 drivers as being available.
I’m not sure what broke or when, but I’m blaming the Apple Software Update program. It’s a pretty simple fix but it’s annoying as it means I’ve been running out of date drivers for a couple of years on this box, despite regularly checking for updates.