John, Michal and 0709
I thank you for your interest and suggestions.
Please let me intervene at this point: I tried John's suggestion to uncheck Navigation>Auto-recalculation. It works to a degree: the start point is now what I entered. The grey box that appears on the left of the screen and towards the top has a question mark in it rather than the distance to the first turn. The itinerary, however, is complete and the distance correct. Two points, however: 1. 'Distance to target' is incorrect and 2. the line defining the navigation route is not always as I have specified - it might be a much narrower and fainter line, for example, either for part or all of its length - almost as though the current location is still trying to have a say in the matter. This suggests the operation is not the proper answer.
I shall read in detail now what you all of you have written and post again in due course. If, in the meantime you have any further thoughts, I should welcome them, of course.