To qoute the docs directly, " After reset, if P12 pin is held high (i.e. connect it to the 3V3 output pin), the heartbeat LED will begin flashing orange slowly. If after 3 seconds the pin is still held high..." suggesting that the pin should be pulled high AFTER reset.
In fact I did get a flashing orange led, using this method.
My problem was that it appeared to reset once I released P12. I released P12 because I did not want to go to the prvious OTA firware and only wanted to disable boot.py and main.py.
I am sure that I have P12 (and not GPIO12 or '12') as I get the flashing led..
I haven't solved it yet. Just documenting for posterity.