The thing I still cannot remotely understand is why Excel was anywhere near the UK testing system in the first place. It's spreadsheet software, not database software – it should not have been within 100 miles of a system developed in 2020.

Please note the operative word in the above tweet is "should". I am well aware of how frequently spreadsheets are used when they probably should not be.

Common practice and good practice are often only coincidentally related, at best.

Also as a safety note it's not yet clear it's actually an excel issue – it could be a filesize issue, or some other similar one – it's not actually been confirmed this is what happened. But any error of this sort on an issue this important is absolutely mortifying.