A clamping mechanism (30) for clamping a flexible tube comprises two rotatable bobbins (22, 32), each with a tube-engaging surface portion (24) defining boundaries of a free space through which a tube may extend. The shape of the tube-engaging surface portion (24) changes around the bobbin circumference, such that axial rotation of the bobbins (22, 32) reduces the free space and thereby squeezes the tube. This allows the flow through the tube to be altered dependent on the rotation of the bobbins.