A drilling system including a drill string (12) which extends into a borehole (10), and a well closure system which contains fluid in the annular space (16) in the borehole around the drill string, the well closure system having a side bore whereby controlled flow of fluid out of the annular space in the borehole around the drill string is permitted, the side bore being connected to fluid return line (28) which extends from the side bore to a fluid reservoir (34), there being provided in the fluid return line a valve (30a) which is operable to restrict flow of fluid along the fluid return line to variable extent, and a flow meter (32) operable to measure the rate of flow of fluid along the fluid return line, the flow meter being located between the valve and the side bore, wherein a filter (40) is provided between the flow meter and the side bore, the filter including a plurality of apertures which have a smaller cross-sectional area than the smallest fluid flow lines in the flow meter.