I have it working again, but I have no idea why!
To resolve this, I opened System Preferences→Shortcuts→Services and just randomly clicked around. I changed one. non-conflicting shortcut key.
As soon as I closed the window and tried instant-send (I'm nostalgic!), *it worked!!!*
No idea why this works, but leaving it here if it's any use for anyone else, as it's a non-obvious solution (and may not even be the solution that actually fixed it, but it's worth a shot!)