I also found the information a bit confusing before my first visit. It states that new users can only check availability online but when I got there they said I could have booked. I ended up sending an email but the best thing might be to ring up, particularly if you have reference numbers of documents that you might want to view etc.
If you have a reader ticket from another archive, you can use it there. If not, you will need some ID with a signature - e.g. driving licence, bank card etc.
It was fairly quite last time I went and might be less busy now that the parish registers are available online.
If you drive, get there early for free parking at the archives. There are car parks nearby.
They seem to prefer you to use digital photography if you can, particularly as photocopies take up to four weeks to process and can be expensive. Having said that, photocopies are better.
Hope this helps,
David