I found out why I couldn't book a Wednedsay appointment.
They are doing Pfizer and AZ on alternate days, as I'd had AZ for the 2 main ones I had to have a pfizer booster.
I guess that they are not showing on the 'book-your-jab' website because they are concentrating on calling back those who had their main vaccinations there?
It was busy and they were running out of space to distance people.