a detection assembly comprising : a body portion having a slot formed along at least a portion of a length thereof , the slot having a slot opening formed in an outer surface of the body portion , the slot opening being arranged to receive a sensor optical fibre through the slot opening ; a sensor optical fibre constrained to lie in said slot and in juxtaposition with a plurality of protrusions ; and at least one swell member , the swell member being configured to increase in volume in response to exposure to a target measurand , the detection assembly being arranged whereby an increase in a volume of said swell member causes said sensor optical fibre to be urged against at least one of said plurality of protrusions thereby to cause bending of said sensor optical fibre .