I tried uninstalling TortoiseGit and re-installing but it didn't fix the problem :-(
FWIW git itself appears to be working - it's just that TGitCache won't auto-launch any more and (even if I launch it manually) I don't see the usual folder icons any more.
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\ShellIconOverlayIdentifiers
I soon realised that Dropbox has cottoned onto the fact that
by prefacing its own entries with whitespace it can take precedence
over the others! I removed the leading spaces from my Dropbox
entries and re-booted. Presto! I now see the TortoiseGit icons
again.