You can't prove you made the downpayment. And they can't prove that you didn't. But the fact they admit they have "lost" the downpayment amounts to an admission that they received it, but lost it, so I don't think that's any different from them losing a £2,000 cash payment. In court, you may be required to show where the money came from that you used to buy the money order, and whoever sold you the money order may be required to show their records for that day which will reveal whether they issued a money order for £2,000 that day and to whom.