Fixed the uploading issue after a PC reboot. That was clearly the cause. My filename procedure now works as it should.
EDIT 5 mins later: Premature - it's not solved!
It worked on first test. But then tried another old board, with the blink sketch as the unavoidable 'base' sketch also open. Same 'programmer is not responding' error with that, and with all subsequent sketches. Including the one that ran correctly on the first test. It seems that the procedure itself is somehow introducing a port problem which persists until a PC reboot.