SOLVED
(Hopefully this is useful to someone else one day…)
Thanks to https://stackoverflow.com/a/66018565/763419 - what I didn't realise was that particular Symfony code being called is a sign that the PHP mbstring package is unavailable.
So I then did dpkg -l | grep 'mbstring'
ii php-mbstring 2:8.1+92+0~20220117.43+debian11~1.gbpe0d14e all MBSTRING module for PHP [default]
ii php7.0-mbstring 7.0.33-57+0~20211119.61+debian11~1.gbp5d8ba5 amd64 MBSTRING module for PHP
ii php7.2-mbstring 7.2.34-28+0~20211119.67+debian11~1.gbpf24e81 amd64 MBSTRING module for PHP
ii php7.3-mbstring 7.3.33-1+0~20211119.91+debian11~1.gbp618351 amd64 MBSTRING module for PHP
ii php7.4-mbstring 7.4.27-1+0~20211220.57+debian11~1.gbp832572 amd64 MBSTRING module for PHP
rc php8.0-mbstring 8.0.13-1+0~20211122.27+debian11~1.gbp1675bc amd64 MBSTRING module for PHP
ii php8.1-mbstring 8.1.1-1+0~20211220.11+debian11~1.gbp519568 amd64 MBSTRING module for PHP
Debian has, for some reason, removed it, fixed with:
sudo apt-get install php8.0-mbstring