I think I'm going to get it directly from O2 now. I think it's usually safer getting it direct from the network operator. I know that 99% of the time third party sellers are ok, but unfortunately I was part of the 1%
I will be finding the best deal I can online and taking a printout of it into the shop and threaten to go somewhere else if they can't match it though. In the shopping centre near me O2, Vodafone, Virgin Mobile, T-Mobile, Orange, Carphone Warehouse and Phones4U are all within 50 metres of each other so they can't risk the chance I'm blagging (it worked last time I upgraded!).