Not sure about this, but....

I don't remember renewing my licence this year. I got what I thought
was a renewal reminder in the post some weeks ago and knowing when it
was due (1st Sept), I left it unopened until today to open it and renew
it, except....

It's not a reminder, rather it is a Payment Plan, suggesting they will
just take the annual payment. In other words they just help themselves
to your money, which I don't ever remember giving them permission to
do.