Hi. Unless it stated specifically on the order form for the car that you would pay him the balance within 10 days, he has broken his contract. If there is indeed an order form, this is to remove the car from sale and only clauses such as this, written in, would enable him to be able to sell the car to someone else.
As an example - If he 'said' to you that he would give you a set of mats and didn't do it, without the written proof you couldn't claim the free mats. Same thing if you said it would take 10 days...
The strongly worded letter would be good as would a little chat with your local CAB...
As a thought, isn't there another car in his stock that would suit your needs? He would prefer to transfer the deposit to another car than lose your custom (this is if you can bring yourself to do more business with him...?).
JJ
You gotta tie yourself to the mast my friend, and the storm will end.