OK - Chromium finally crashed and downgrading immediately to nvidia-280 brought it back to life. I've not even rebooted, just restarted X (which did not run), rmmod nvidia, modprobe nvidia then restarted X again, and it's all good.
Not sure what I think about comments about who is in the wrong here, chromium or nvidia. chromium is open-source, nvidia is not, therefore can the developers of chromium see what's changed in the module that causes the problem? The only justification I can see for keeping nvidia-285 is if it fixed other things or made other cards work. Maybe it's time for a legacy split-off again?