Eureka. Don't know what happened to your other post but, in any case, your answer was perfect. I'm new to the site and delighted to get such a speedy response. My situation was similar to the 2011 one but next time I'll do as you say.
As there is no indication of which black is live and which not I have tried connecting one and then the other to the neutral lead to the bulb holder. In one case nothing happens; in the other the lamp comes on but can't be turned off. What have I done wrong?