If you've deposited the money into a solicitor's trust account, getting your money back should be straightforward.  Most contracts have a sunset clause that allows the buyer to call off the sale and reclaim their deposit if the property is not completed within the allowed timeframe.

Read your contract and find the sunset clause (they might be required in contracts of off the plan sales) then contac the developer's solicitor.  As the money would be normally held in his account, it should be as simple as providing a written notice that you want to cancel the contract and want the deposit back.

Hope that helps.