In 1765 Cox went into partnership with Henry Drummond, whose family ran the London bank. Cox & Drummond moved from Cox's house in Albemarle Street to Craig's Court, just off what is now Whitehall. By the mid-1760s Cox & Drummond had blossomed to become agents for the Dragoons and eight more Infantry regiments. Success was built on the company's reputation for keen attention to the welfare of its regiments. In 1763, for instance, when Robert Clive stormed the fortress of Gheria in India, Cox successfully negotiated with the East India Company who had "borrowed" stores from Cox's clients, the Royal Artillery. He arranged to receive repayment from the East India Company by way of plunder from Gheria. He had this converted into silver in India and shipped back to London where the funds were returned to the Royal Artillery.