That would have been OK, but this part of the route is too far from the next part. I don't think ID can handle this. Potlatch2, suggested by Mike Baggaley, is the answer as you can select any relation, not just nearby ones.

That is so simple. I think it's impossible in ID, and JOSM is just too confusing that I don't know whether it's possible or not. All done now until our next walk. Thanks. David

I wish I knew how you had done that! It's a good job I made the new relation a 'Hiking Route' and not a 'Foot Route' or I wouldn't know which was which when selecting the relation in ID editor. Thanks, David

That is my plan, but I cannot see a way to do it yet, which is why I added a note to the map asking for help. Once I complete a bit more of the route between the section I have done and the existing section, it will be reasonably easy to change the relations, but until then I am stuck. I am sure there is a way to do this because there are already some other separate sections, but I cannot find an explanation of how to do it that actually works.