When buying a new build property off plan and paying a reservation fee to the developer this will allow the buyer to reserve a property for a set period, usually 28 days in order that they and their solicitor can carry out the necessary due diligence and exchange contracts. There should be a written agreement setting out the terms of the reservation.
Sometimes the reservation fee is stated to be non refundable if the buyer fails to exchange contracts within the specified time period. To protect your interests you should have requested your solicitor to review the agreement before you entered into it and paid a reservation fee. Your solicitor would have ensured that the fee was not stated to be non refundable and that in the event of you failing to eschange contracts the developer could only retain reasonable administration expenses from the reservation fee.
Frequently developers are register with a home warranty provider such as the National House Building Council. If so the develper would have agreed to comply with the Consumer Code for Home Builders. This would mean that they can retain reasonable administration expenses but the reservation fee has to be refunded.