A clamp (10) for holding two members (12, 14) together. The first member includes a recess (30) into which a fastener (38) is located. A further access port (42) is provided to the recess to allow a tool (44) to be used to operate the fastener. The second member is arranged to accept the fastener and so join the members. The recess and port are sized to prevent release of the fastener through the port. In this way, the fastener cannot easily be removed or lost when the clamp is in use and is tamperproof. Embodiments of a downhole tool are described where the clamp is used to hold portions of a sleeve around a body of the tool, and to hold scraper blades (384) and/or brushes (394) to the body.