I know it wont help you now, but in the future I'd recommend buying from carphone warehouse. Don't know if it's just in my area, but they have the most knowledgeable, no-bs staff and they sell nearly everything unlocked. Even the contracts they sell are sold with unlocked and unbranded devices.
Impulse buying and taking the word of a store salesman is a recipe for stress.
My advice to other people would be this: research first *very* thoroughly. Seeing as it was a £579 iPad and not a portion of cod & chips, and seeing as it has cost you all that time and stress, then I am bemused that you simply strolled into the store, asked some sales assistant "is it unlocked?", they say "yeah, tis" and you handed over nearly 600 smackers?
I sympathise, having skimmed over this story, but I would also say you are also responsible for not having done your research, looking at the considerable cost of the item.