I might try something like you suggest, when I next have a couple of hours to spare.
In the meantime, I'll simply confirm that the site does know that I am an over-65, because the Price Type of 65+ is what is stated against my name. The option of changing this isn't available.
Trying to buy tickets for Roz and myself, I successfully logged on and selected the two tickets I wanted: mine at the 65+ price and Roz's at the adult price. I then clicked "proceed". Up popped the following message:-
You have selected an invalid number of tickets either for this ticket owner...