Thanks for all the great tips! I need to practice more...
You're very welcome, MeeMaw, I've received a lot of inspiration and encouragement from you over the years. Any skill I have with the clone tool comes after years of practice; when I started out I was hopeless with it and almost despaired of ever getting satisfying results.
These days I can get results good enough to get by with but nothing is really as satisfying as a clean photograph of the original. I've tried, over the years, the technique of taking two or more photos from a slightly different perspective to eliminate such things as power-lines, road signs and other obstructions. I still haven't perfected this technique and need to learn more about realigning photos that have slightly different perspectives. I've read one tutorial on using Hugin to do that but haven't yet succeeded. If you were to use this technique combined with retaining the intermediate TIF files that Hugin produces and editing those with GIMP to remove any unwanted features, then I think you might get the best results possible.
The combining multiple-photos technique can also be used with people moving in the foreground of a scene that you want and by taking several photos with the people in different places, meaning that you have all the background in the combined photos, you can eliminate the people altogether using masking or other removal techniques. Hugin on its own can 'ghost' moving people and so combined with another step in GIMP to remove them it should produce clear scenes if there is sufficient overlapping data.
One of the limitations of my work, so far, is that it is all done hand-held and I need to invest in a tripod. I located one at a price that I could afford but when I went to the shop to buy it they didn't have it in stock or on back-order, so it has been back to square one for now.