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Please enable Google apps location access
That was all I had to go on. No link to perform said action, just a temporary on-screen notification which did nothing when pressed.
I checked in Settings → Location access and everything looked fine. I rebooted and it made no difference. Twitter could still access the GPS so it wasn’t a hardware problem.
Turns out there’s a new toggle in Settings → Accounts → Google → Location settings which is off by default and needs to be on.
So Google have pushed out a release which disables GPS and creates a new, non-obvious, toggle to enable it. Hopefully this post helps someone who hits the same problem.