I checked the electrics and, on the face of it, everything seemed to be in good order. The points had the correct gap - 15 thou - and were opening at the right time. So what was it? Not another condenser? I replaced it just in case - but it made no difference.
Then I recalled, many years ago, reading an article which suggested reducing the points gap to 8 thou. So I did - and, to my surprise, it worked and the little D1 now starts easily and runs (I almost said 'purrs') beautifully. She actually pulls away much faster than my D7, Little Willie, so I think the latter may be due for a rebore and new piston! Something else to do.