Perhaps beyond the remit of this thread, but what I don't really understand is why, with a seemingly highly effective TT system which outperforms so many other countries, we underperform so badly in suppressing the infection rate and its sad corollaries.

That is probably really very simple.
Far too many people just are not being sensible enough to prevent cross-infection.
There really can be no other logical explanation.

I'm not a fan of anecdotal evidence as you have probably noticed, but simply looking around you (when of course you are on an essential journey) will show that theory to have considerable merit.

Baroness Harding said in a report to Parliament's science committee that the act of tracking down people who have been close to Covid-19 cases has only a 'relatively small' effect on the R rate of the virus.

The letter, published today, estimates that the UK's entire test, trace and isolate system would keep the R down by between 18 and 33 per cent in a scenario similar to October 2020, when there were an average of 19,000 cases per day.

It became less effective when the outbreak got bigger in November, December and January, and when the fast-spreading new variant took over, Baroness Harding admitted.

She said: 'The impact of contact tracing alone reduced the R number by 2-5% (with testing and self-isolation accounting for the remaining 16-28%).'

In a report published alongside the letter Test and Trace chiefs said it was unlikely the service would improve on this, saying: 'The impact of contact tracing is relatively small. Even with small changes or improvements to the model, it is not expected that the impact of contact tracing will change drastically. It would remain of the same order of magnitude.'

The admission led critics to question whether NHS Test and Trace's eye-watering £22billion budget is good value for money.