Improve test coverage

[jfrost-mo]

Various improvements to improve test coverage.

Fixes #708

Contribution checklist

Aim to have all relevant checks ticked off before merging. See the developer's guide for more detail.

• [ ] Documentation has been updated to reflect change.
• [ ] New code has tests, and affected old tests have been updated.
• [ ] All tests and CI checks pass.
• [ ] Ensured the pull request title is descriptive.
• [ ] Conda lock files have been updated if dependencies have changed.
• [ ] Attributed any Generative AI, such as GitHub Copilot, used in this PR.
• [ ] Marked the PR as ready to review.

[github-actions[bot]]

[jfrost-mo]

TBH it is probably best to split this PR into a few, so as to make them easier to review. Some commits, like 05d82f0a9a286b8b9b5934f092680a5d1a9cec40, require specific expertise.