OK. More bad news. It's not so much about the sale dragging on, but, rather, about the vendor getting another offer, or the mortgage offer expiring.
When we bought the house we live in now my one spark of inspiration was to talk directly to the vendor. I wanted the house and she wanted the money. We both realised that the way to achieve this was to make sure that the 'professionals' were doing their job, and it transpired that her solicitors were a sack of shite. Happily I could report every move that our solicitors made, and every move the surveyor made, and, by doing that, it eliminated the 'we're waiting for the purchaser's solicitor to get in touch' rubbish.
The estate agent loved us. He really did - we were doing his job, but able to do it better than he could, because we were speaking to people that we were paying. And he, the agent, just wanted the sale to go through.
On the down side I think you have a responsibility to be honest with the vendor. They may be just as anxious about this as you are, and when it looked like our sale would fall through (Foxtons had the nerve to indulge the purchaser in a bit of gazundering) the first thing we did was to tell the vendor that we were adrift. But, even that worked out, because the estate agents talked to each other, and realised that we were seriously not going to suffer a £20,000 last minute reduction in the price.
So, my advice is a kind of extension of Crankarm's words of wisdom above - go and see the house as often as you can, and build an honest dialogue with the vendor.