Thanks everyone. It is petrol not diesel, and as the trouble came on suddenly one day for no apparent reason about six weeks ago after years of reliable starting in all weathers I don't think damp is causing the problem.
I suspect its something that has failed, and it is something that only affects cold starting. The effect is that it turns over briefly with no sign of firing. You try again with a foot on the gas and it fires. If you immediately remove your foot it stops, but perhaps three or four seconds of moderate reving (1,500 rpm maybe?) you can take your foot off and it idles perfectly sweetly at 1,000 as usual.
It's pretty old and dates back to 2003, so its not worth spending serious money on curing what seems to be a tolerable irritation. But if a simple solution is possible I'll take it. I will look into the wirral spark centre though.