Hi Lynn,
The answer should be that you are able to make them adhere to the agreed plans which are part of the contract. However, there may be terms in the contract that allow the developer to change the plan or specification without consulting you. You need to check this with your solicitor. Barratt Homes recently got pulled up by the Office of Fair Trading for having unfair contractual terms - see:
Barratt Homes OFT Complaint
Secondly, they are probably in breech of the property misdesciption act. Only trading standards can take action under the act, but you could take action civil action making a claim based on their misrepresentation.
Unfortunately all this will take time and money in terms of solicitor fees. You should speak to your conveyancing solicitor too - they should be able to help, but will probably only take it so far without your having to pay additional fees.
These developers have little respect for the law or their customers.
I hope this helps. Good luck and let us know how you get on.
Tony