It could be they have been caught up in the tightening mortgage situation, but 14 weeks is pushing it. Your estate agents assume you will wait until this mortgage rolls in under its own steam, but it is you who instruct them and pay them and they cannot assume they have sold your house under these circumstances. Until the money changes hands it is not sold. Agents often order the sold sign prematurely.
You need to remind the agents that you are their client, give your buyers a deadline - a tight one - and stick to it. Or put it back on the market today. You may lose these buyers. Have they had a survey done? The risk of losing some money may get them moving. I know its tough and you dont want to scare them off but are they really doing you any good? It is a sticky situation out there at the moment and another buyer may not be so quick onto the scene. But I think you have been patient enough and your suspicions, and those of your solicitor, should move you into some action.