This questions have been asked many times before.
A mortage offer is generaly valid for 3/6 months. You have no recourse towards the bank.
Chances are, if you are reapproved, you will not be offered the same amount of money.
Your options are:
1. Walk a way and lose at least your deposit, the developer can sue you for not fulfilling your contract. There are a number of these cases in the courts at the moment, but I have yet to hear any outcomes.
2. Offer the developer the reduced mortgage the bank offers you.
3. If 2. is refused, I have heard (not confirmed) of someone getting in contact with the developers bank and buying the house (at the reduced rate) off the bank, cutting out the developer.