Here's the moral of this story. Never legally commit yourself to buy a property by exchanging contracts unless you can be sure you will be ready, willing and able to complete the purchase on the date which will be specified in the contract.
If you can't complete when the contract says you should, the seller can send you what is called a notice to complete. It will set a final deadline for you to produce the cash and that could be as close to five working days from when you get the notice. Still fail to come up with the cash and what will happen? The seller can cancel the contract and probably keep your deposit. Ouch?
In a case just before the High Court, the buyers had paid a deposit of a cool £43,000. One week before when completion was required they told the seller that they might be late because of delay in funds coming through from a Nigerian bank account. They didn't take up offers by the seller to put the completion date back in return for a modest price increase. The completion date arrived and they weren't ready. The seller sent them the dreaded notice to complete. The final deadline passed without completion. So the seller cancelled the contract and told them they were keeping the deposit. The next day the buyers were ready with all the cash they needed!
The buyers' attempt at getting the £43,000 returned to them through the court has failed despite the fact that the seller later sold the property for an extra £100,000 (albeit with the benefit of planning permission although it had been known that planning permission was on the cards when the original buyers agree to go ahead).
The key to having a decent chance of securing a deposit return is to come up with some good proposals to compensate for lateness, to keep the seller very closely informed, not to be very late and, with a bit of luck, to be able to rely on some delay on the part of the seller (for example, in answering queries after contracts have been exchanged or in providing relevant details). Any extra money the seller later managed to obtain when selling to someone else will be taken into account but as just one of the considerations.
The High Court case was called Solid Rock Investments UK Ltd v Reddy and another  EWHC 3043 (Ch).