at tune sessions, where most of us play tunes that everyone can join in with there is a tune that I introduced to a friend (we used to have a home practise session), she introduced it to a couple who she played with. When they then started playing it at sessions in an arrangement that no-one else could join with I did feel a bit miffed

I would welcome that. If somebody can play a tune I know well in such a way that I couldn't possibly join in, it's in no danger of being ossified. The session scene often murders tunes by pointlessly standardizing them.