Flyspell in Emacs 26.1 seems to require a version of Aspell that is not officially available for Windows. I might need to put off the upgrade until I can attempt to compile a newer Aspell for Windows myself.
"Error enabling Flyspell mode: (aspell release 0.60 or greater is required)"
In case anyone else was facing this problem (and isn't too committed to Aspell as a backend for Flyspell), the version of Hunspell from ezwinports[1] (version 1.3.2 at time of posting) seems to be working with Emacs 26.1 on Windows.
It works flawlessly. I just dropped it (26.1) in on top of my existing setup and had to do nothing else and I'm working away on it right now.
When I was downloading at least, I had to download it and the dependencies separately. The README says there should be a unified zip-file but it wasn't there at the time.
Had some issues with Windows 10 refusing to merge the directories from the deps package so had to drop to a cygwin shell but it actually worked quite well even without the deps.