A drilling method comprising running a drill string (12) part way into a bore (11); then pumping fluid through the drill string; then running the drill string further into the bore; and then reconfiguring a check valve (20) located towards the distal end of the drill string from a running configuration, in which the valve permits flow both up and down the string, to a drilling configuration in which the valve permits flow down through the string but prevents flow up through the string.