Release v4.2.1

[jemrobinson]

✅ Checklist

• You have given your pull request a meaningful title (e.g. Enable foobar integration rather than 515 foobar).
• You are targeting the appropriate branch. If you're not certain which one this is, it should be develop.
• Your branch is up-to-date with the target branch (it probably was when you started, but it may have changed since then).

🚦 Depends on

n/a

⤴️ Summary

Release candidate for v4.2.1

🌂 Related issues

Closes #1914

🔬 Tests

Details in #1914

[jemrobinson]

@JimMadge : I'll take responsibility for merging this (and backporting to develop) once the TRESA team are happy with it.