@davidhassell please either hold on the reviewing here until tomorrow morning, or you can partially review under the following context:

• as-is on this branch, test_aggregate passes fully, but notably I've commented out a set of related test assertions, which rely on a fix in cfdm for which I am preparing a PR but it is not quite ready so I will put up tomorrow morning;
• once the corresponding cfdm PR is in, I will self-review that everything's working together and then open this for review fully.

If you like you could review the commits here standalone noting the above, with the main one is the initial tweak to fix an aspect of Data.equals, 02133f7.

Thanks for all of your invaluable guidance here @davidhassell. (I manually added you as a co-author on the new fix approach commit, since you essentially wrote that code!)

This is good to review now, I think. FYI, the linting CI job is failing due to steup issues relating to GitLab (I see these on the command line too, so think it will resolve itself in time and will ignore it for now). So we can ignore that.