Generally, the law would side with consumers because they are considered the weaker side with fewer bargaining powers, but it is not always in their favour.
When a person places an order for something and pays a deposit they enter into a legally enforceable contract with the other side. It is implied that they have accepted the deposit as security and as proof that the customer wants to proceed with the contract.
Unless the provider (you) subsequently commits a serious breach of contract, such as failing to supply the goods or services, or they are the ones cancelling the contract, or there was a cancellation clause, the customer would have no legal right to cancel the agreement. If they do so they will likely be acting in breach of contract and risk losing their deposit. This is reinforced if the contract or initial agreement said that the deposit was non-refundable. Even if this was not explicitly mentioned it is entirely possible for a deposit to be retained if the contract has been cancelled when there was no right to do so.
Whilst there is nothing specific in law which would allow the customer to demand the return of their deposit, they could try and argue that you are basically profiting from them when you may not have suffered any losses. This would all depend on what happens with the caravan - if you manage to sell it at a similar price and recoup any potential losses then that would be a much stronger argument, but if you have to take a hit on it to shift it - then you could potentially keep part or all of the deposit to cover such losses.
But my recommendation is not to automatically decide to keep it, but inform her that this is done temporarily for the time being to cover any potential losses and once you know what these may be, you will reconsider the position and if she is due anything back.
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