Techniques are disclosed to aid fixing of an elongate wire within an elongate, linear cell of a honeycomb ceramic substrate unit for a gaseous emissions treatment assembly. In one method, the wire is formed with a resiliently flexible element, and inserted into the cell, the insertion act causing the resiliently flexible element to flex and to cause a part of the element to bear against a wall of the cell and so provide frictional retention of the wire in the cell. In another, method, an adhesive is used either on the outside of the wire or the inside of the cell. In another method, the wire is scored at spaced intervals along its length to provide relief spaces for linear expansion of the wire to reduce stress at its interface with cell walls.