OK - not to answer my own question, but after a lot of messing about, I realised that the only way to do this was to have a separate calendar, which I could assign the new fifth appointment type to, and then I could block off the days I didn't want on there.
So I stumped up the additional money to move from the basic "Emerging" Acuity plan to the next level "Growning" plan which allows up to six calendars. Not ideal - but hey, I can now do what I want to do. Thanks!
