buying a new build property - 28 days exchange of contracts?

I've placed a deposit on a new build property (in the UK) which won't be ready until October/November. They set a 28 day deadline to exchange contracts. I've had the survey + legal searches and have a mortgage offer coming through. The deadline is next week, and the buyers at the bottom of my chain have pulled out today. I could be stuck for at least another month. Are builders normally flexible when this happens?