The independent Covid Symptom Study app from Kings College, already on 3 million UK phones since March, took three days to write and largely would do that if it was more publicised by the government.

The NHSX app tried to do too much — trying to collect centralised data when one key function didn't need to be centralised was always going to be an issue when trying to get people to install it in large numbers.

Smaller samples are enough for population surveys, while high percentages are needed for the track and trace function. Mixing up the two into one app has doomed this project.