I think this is probably a glitch on their site but I've phoned O2 and they have confirmed the details of my tariff for me as being the above.
Basically the normal offer is for 600 mins and 1200 texts (as is already a deal on here) on a 1 month contract. If you go through the online shop, you'll see that the 800 min 1600 text deal shows up as a 12 month contract...
HOWEVER... if you go through the Upgrade Online procedure like I did on Saturday you'll notice that both the 600 and 800 min contracts show up as 1 month!
I phoned O2 once I'd completed my upgrade to double check I was going to get what I'd signed up for on the site and the man didn't seem to think so (but he couldn't check until the transaction had fully gone through - til today) so I phoned up again just now and she confirmed that I was on a 1 month contract for the 800 mins and 1600 texts.
You can't get this deal through over the telephone and as far as I know it'll only work if you "Upgrade Online'
Good luck to everyone!