A valve for use in a fluid system, the valve comprising: a housing having an inlet passage (P I), an outlet passage (P O) axially spaced apart from the inlet passage (P I) relative to a longitudinal axis extending through the housing, and a pressure reference port located axially between the inlet passage (P I) and the outlet passage (P O), the housing including: an inner-housing wall enclosing a housing cavity providing fluid communication between the inlet passage (P I) and the outlet passage (P O); a rim in the housing cavity extending circumferentially about the longitudinal axis, the rim defining an axially-extending bypass passage (P A) circumscribed radially outwardly by the inner-housing wall and radially inwardly by the rim, a radially-extending upstream passage (P Ra) located axially between an upstream end of the housing cavity and the rim, and a radially-extending downstream passage (P Rb) located axially between the rim and a downstream end of the housing cavity, wherein the inlet passage (P I), the upstream passage (P Ra), the bypass passage (P A), the downstream passage (P Rb) and the outlet passage (P O) are in serial flow communication so as to define a flow path through the valve; a chamber circumscribed radially outwardly by the rim, the chamber in fluid communication with the pressure reference port; a rib extending radially outwardly from the rim to the inner-housing wall, through the bypass passage (P A); and a piston assembly received within the chamber, the piston assembly including a piston movable axially relative to the rim between a first valve position in which the piston obstructs the flow path at the upstream passage (P Ra) and a second valve position in which the piston is at least partially clear of the flow path to permit fluid flow along the flow path, wherein the housing includes a body defining the inlet passage (P I), characterised in that: the housing includes a transfer tube defining the outlet passage (P O), the transfer tube slidably received by the body; and the axial length of the valve is selectively variable, wherein means are provided to selectively position the transfer tube relative to the body. a housing having an inlet passage (P I), an outlet passage (P O) axially spaced apart from the inlet passage (P I) relative to a longitudinal axis extending through the housing, and a pressure reference port located axially between the inlet passage (P I) and the outlet passage (P O), the housing including: an inner-housing wall enclosing a housing cavity providing fluid communication between the inlet passage (P I) and the outlet passage (P O); a rim in the housing cavity extending circumferentially about the longitudinal axis, the rim defining an axially-extending bypass passage (P A) circumscribed radially outwardly by the inner-housing wall and radially inwardly by the rim, a radially-extending upstream passage (P Ra) located axially between an upstream end of the housing cavity and the rim, and a radially-extending downstream passage (P Rb) located axially between the rim and a downstream end of the housing cavity, wherein the inlet passage (P I), the upstream passage (P Ra), the bypass passage (P A), the downstream passage (P Rb) and the outlet passage (P O) are in serial flow communication so as to define a flow path through the valve; a chamber circumscribed radially outwardly by the rim, the chamber in fluid communication with the pressure reference port; a rib extending radially outwardly from the rim to the inner-housing wall, through the bypass passage (P A); and an inner-housing wall enclosing a housing cavity providing fluid communication between the inlet passage (P I) and the outlet passage (P O); a rim in the housing cavity extending circumferentially about the longitudinal axis, the rim defining an axially-extending bypass passage (P A) circumscribed radially outwardly by the inner-housing wall and radially inwardly by the rim, a radially-extending upstream passage (P Ra) located axially between an upstream end of the housing cavity and the rim, and a radially-extending downstream passage (P Rb) located axially between the rim and a downstream end of the housing cavity, wherein the inlet passage (P I), the upstream passage (P Ra), the bypass passage (P A), the downstream passage (P Rb) and the outlet passage (P O) are in serial flow communication so as to define a flow path through the valve; a chamber circumscribed radially outwardly by the rim, the chamber in fluid communication with the pressure reference port; a rib extending radially outwardly from the rim to the inner-housing wall, through the bypass passage (P A); and a piston assembly received within the chamber, the piston assembly including a piston movable axially relative to the rim between a first valve position in which the piston obstructs the flow path at the upstream passage (P Ra) and a second valve position in which the piston is at least partially clear of the flow path to permit fluid flow along the flow path, wherein the housing includes a body defining the inlet passage (P I), characterised in that: the housing includes a transfer tube defining the outlet passage (P O), the transfer tube slidably received by the body; and the axial length of the valve is selectively variable, wherein means are provided to selectively position the transfer tube relative to the body. the housing includes a transfer tube defining the outlet passage (P O), the transfer tube slidably received by the body; and the axial length of the valve is selectively variable, wherein means are provided to selectively position the transfer tube relative to the body.