What exactly was the arrangement - verbal? There is no problem necessarily with a verbal contract - it would just be better if you had written to him with the quote.
I would write to him, stating that you agreed a price of X, that you have now completed the wall, and that you have requested payment and that you understand he is refusing to pay on the grounds that the price is too high (repeat what he said back to him - this gives him the chance the challenge you). The important thing is that he is not questioning the quality or workmanship of what you did - and you want to get this established in your letter to him.
Assuming he does not respond to this, then use the Small Claims Proceudre in the County Court to extract your money.