a control arrangement for use in a medical or surgical instrument , which arrangement includes a keypad for providing manual control functions to a user of the instrument , a retainer clip and a sealing member . the keypad and retainer clip are located within an opening formed in the instrument housing , whereby when the retainer clip is assembled to the housing , the retainer clip automatically causes the sealing member to sealingly engage with a corresponding sealing surface defined adjacent the housing opening , without the need for the use of additional sealing agents .