GMgm3dmo·Jan 5, 2021
https://www.notifications.service.gov.uk/
and
https://www.payments.service.gov.uk/
are both open source:
https://github.com/alphagov?q=pay&type=&language=
https://github.com/alphagov/notifications-api
In Busiest hour ever in the history of Gov.uk 17M page requests just 21 5xx errors