Same as Kiddo - when I asked about it at my 24 week appt, it turns out that in my original pack was a list of dates - I don't need to book, just turn up. And she told me which dates to go to to fit in with my pregnancy.
I've also looked in to private ones - found some small, cheap ones locally which I'm going to try.
Also going to a monthly NCT homebirth meeting for the first time on Monday - the private classes have an emphasis on homebirth too and it's what I'd ideally like, so am trying to get as informed as possible.
I was in two minds: ignore it be totally unprepared, or gen up so I know what to expect. Have tentatively chosen the second option.