Resisting leakage from a downhole tool string

There is provided a downhole tool string 100 comprising a plurality of generally tubular bodies 102, 104, 106 formed of metal and connection means operable to connect said bodies one to another in an axially extending line; wherein at least one said body contains a flapper check valve assembly 114 with a valve housing 116 that is a sliding fit in said one body; wherein the valve housing and said one body are formed with two mutually opposed annular faces, one on the valve housing and one on said one body, that are caused to approach one another by operation of the connection means to connect said bodies; and wherein there is between said two faces a deformable ring of metal that is compressed between said faces when the connection means is operated to connect said bodies and thereby forms a seal 130 in the tool string 100.