I think you may have misunderstood your contract, helped along by some shop goon not explaining it properly. Your contract will state that you took your tariff and phone along with a data bolt on, but this data bolt would always have had a £5 charge (listed separately, but not actually a separate item which is where the confusion comes in) as the only tariffs that exist with data pre-bundled since feb 2010 are the iphone tariffs. As adam said, there has never been a 750mb extra bolt on, and the extra bolt ons that do now exist only came out in November, so the contract you descibribed could never have been applied and still cant.
However cs response to your call was, i believe, first class, and giving you a £5 discount you arent actually entitled to just because an idiot in store screwed up is brilliant way to make sure you get what you were told youd get. Remember they didnt do this to you and they are a different department so it was very good of the guy to do this for you instead of just directing you back to the store for them to sort out their own mess ( different departments are responsible for their own budgets and answerable to higher management for giving too much away). In your place i wouldnt be complaining about that, id be full of praise.
I cant believe the discount was only added for 1 month because, as far as i am aware, discounts can only be added for 12, 18 and 24 months ( pre set). I would get them to check tnis for you and if it was done that way im sure they would change it again, as a one month discount would be meaningless.