Not meaning to add more worry to what is an already frustrating situation, but remember to bear in mind that until contracts are signed, the vendors are entitled to accept a higher offer at any time from other interested purchasers. Property prices tend to rise over the summer period, so the property may well be worth more now that it was in May, and your vendors are probably aware of this.
If you finally do get them to agree a closing date, often contracts will include a penalty clause whereby if the purchasers delay completion past the agreed date they pay a penalty (usually expressed as a percentage). I'm not sure whether this penalty works both ways, but maybe other posters can advise...