Sorry to disagree, but that is wrong.....deposits are not refundable, especially if stated as non-refundable, in the case of the client letting you down, or changing their mind.
This statement below is taken from the linked Trading Standards advice sheet on deposits.
"Sometimes when you agree to buy something you may be asked to put some money down to secure the item. This is usually known as a deposit. It means you are entering into a LEGALLY BINDING CONTRACT with the seller for the goods.
It is a sign of good faith you have agreed to make a purchase. If you change your mind for whatever reason, a trader DOES NOT HAVE TO GIVE YOU YOUR DEPOSIT BACK."
(The underlines and caps btw, aren't me shouting....they are in the text in the doc!)
http://www.tradingstandards.gov.uk/kent/factsheets/FS5NEW nov07 refunds.pdf
Hope this helps....it should give you some idea's as to your wording and position.