What became quickly obvious was the source of the smoke, a slow burning ground fire in the composting undergrowth. The cause of the fire was some ash tipped there 12 hours earlier from a small garden wood burner which itself had gone out 16 hours before. The ash was not hot although it did have a little warmth. Clearly enough to start a fire in these dry conditions even though we have just had a night of heavy rain.
We were able to put out the fire with a hosepipe but it served as a useful warning to be more careful about the risk of fires especially in these unusually dry conditions.