The best time to do this is when you re paid. Just send them a note to say thank you for the payment and that you look forward to working with them. If they ve paid promptly, say that you appreciate this.It doesn t have to be a long letter. At one time we used to send a card (that also advertised our business then); more recently we tend just to drop our key contact an e-mail.
Usually in contract work, no letter of Thank You is necessary Your professional work expresses appreciation for the jobYou may write a personal note on the billing such as Thank you for allowing us to work with you and please remember us in the future
I think I would write to the MD thanking him for the opportunity of doing business with them, trusting that your work was to their satisfaction and hopefully that you can continue the business relationship.