I think that in the current climate, everyone is under a lot of pressure, I think people are setting themselves impossible deadlines, and a lot of trust is gone out of the whole thing. As a practising solicitor, I am regularly finding purchasers who said they were first time buyers( ie. no house to sell) and loan approved ( i.e. ready to go) are lying on all counts to get the house they want. So a vendor who maybe agreed a slightly lower price with a "ready to go purchaser" so that they themsleves could commit to their own new purchase, now find that they are stuck in the next part of the chain. I equally have vendors who agree in theory to a quick sale and then try and drag out the deal so that they can find their next house and try and tie in the two deals without any inconvenience to themselves.
To answer the original poster, ask your solicitor to put in writing that the sale must close on the agreed closing date, that a completion notice will be served on the closing date if the sale does not close, that interest will be charged on the closing funds and that you are also reserving your rights to terminate the contract etc.,etc.. But remember, you cannot get money from someone who does not have it and who may have foolishly committed themselves to something they cannot afford. So, you do need to find out precisely what the problem is - are they in trouble with funds, have they got cold feet, do they want to back out? For you to complete on your new place, you could try to get bridging finance because you do have a signed contract but are you happy that the original purchaser will ever close? If they don't , how long will it take to sell the house again?
mf