I totally managed to mess up my Gemfile.lock file setting the versions of many gems to far higher versions than I’d intended. bundle update is bad, even if you set the version in your Gemfile the dependencies of thos gems are not versioned so precisely. I did this in a branch so fortunately the Gemfile.lock in my master branch is accurate.
git checkout master -- Gemfile.lock
That did the job. Then a bundle install puts the changes I’d made to my Gemfile into Gemfile.lock with the correct dependencies as per master. Phew.