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All you can do is get your Solicitor to fax/email your Buyers Solicitors and give them an ultimatum, saying "unless you are in a position to exchange Contracts within the next X days, with completion on 12th November, then our clients may well lose their Purchase property and would therefore have to to withdraw from the Sale to your clients".
I would also strongly suggest that your Agents phone your Buyer directly to confirm the above, and hopefully the Buyer will give their Solicitor a rocket!
I hope this assists you and answers your question.