Basically I want to do a PhD at Sussex in Astronomy. I've wanted to do so for a long time. For a plethora of reasons, Sussex is the best choice for me. I already live in Brighton, girlfriend there, her job's there, all my friends and due to certain events in my life and being forced to live in lots of different places, I've always been the happiest when in Brighton.
I'm currently on a 2:2/2:1. It'll be touch and go whether I end up with a high 2:2 or a 2:1. My tutor is confident that I'm in with a very good chance of a 2:1 right now but its no slam dunk. I had originally planned to do a masters in Astronomy at Sussex with the view to apply internally for a PhD after that. That was under the previous basis of a 2:2.

I originally had an offer for an undergraduate place at Sussex with a Bursary (top top A-levels) but decided to spread my wings and go somewhere else in the country to a Uni higher up those useless league tables. Biggest regret of my life by a long long way and I'm not one to have regrets!!

Now is it worth me applying straight for a PhD even though I might not get the grade I need? Say I did get offered a position and then missed out, do you think they'd let me do the Masters and then reapply for a PhD the following year? Would missing out on a grade one year, getting a masters and then reapplying hurt my chances?

I think you should give it a try. If your referees feel that you will get a 2:1 then you should go for it. If you get good references and they predict that you will get a 2:1 then you will be in with a chance. You could also apply for a masters just in case you dont get the grade then you can top up yr 2:2 and re-apply next year.