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I am afraid there is nothing you can do and there is no point in you wasting your time with the Legal Ombudsman, as the Buyers have acted in accordance with the terms of the Sale Contract.
I agree that it is pretty shocking- I would have understood if the Buyers had had a related sale and needed to move into temporary accomodation, but this wasn't the case.
I am sorry I can't say anything more positive, but your Solicitor is 100% correct.