www.nhs.uk/conditions/coronavirus-covid-19/coronavirus-vaccination/book-coronavirus-vaccination/book-or-manage-a-booster-dose-of-the-coronavirus-covid-19-vaccine/
This has appeared this morning, worked for me -
"Important
If you have not been contacted and it's been 6 months and 1 week (190 days) since your 2nd dose, please try to book your appointment using this service."