A downhole cleaning apparatus comprising: a tubular body having an axial through bore and comprising a plurality of openings therethrough, wherein each opening defines at least part of a helix extending longitudinally and circumferentially about the body; and a plurality of cleaning elements configured to extend through the openings and to extend outwards from an outer surface of the body, wherein the cleaning elements are distributed about the body and each cleaning element defines at least part of a helix extending longitudinally and circumferentially about the body, wherein the openings and cleaning elements are grouped to define a plurality of substantially continuous helical paths, each of which extend substantially end to end of the tubular body and are distributed to define a cleaning surface of at least 360 degrees; wherein the cleaning elements are retractable and extendable, wherein when retracted the cleaning elements are stored within the body until required for use; and when required for use the cleaning elements are extendable to an extended position in which they extend outwards from the outermost surface of the body; wherein the cleaning elements are held in a retracted position by a holding system, comprising one or more shear pins (24, 26, 28), until the cleaning elements are required for use and wherein an activation system is operable to at least prime the cleaning elements for movement from a retracted position to the extended position; the activation system comprising a setting sleeve, internal to the tubular body, and operable to move in an axial direction relative to the tubular body and operable to break or shear one or more of the said shear pins (24, 26, 28) such that the cleaning elements are at least partially primed for movement from the retracted to the extended position; wherein each cleaning element is biased to the extended position and wherein a shear pin prevents, until broken or sheared by a radial load generated by physical contact of the setting sleeve on the cleaning element, movement of the cleaning element from the retracted position to the extended position. a tubular body having an axial through bore and comprising a plurality of openings therethrough, wherein each opening defines at least part of a helix extending longitudinally and circumferentially about the body; and a plurality of cleaning elements configured to extend through the openings and to extend outwards from an outer surface of the body, wherein the cleaning elements are distributed about the body and each cleaning element defines at least part of a helix extending longitudinally and circumferentially about the body, wherein the openings and cleaning elements are grouped to define a plurality of substantially continuous helical paths, each of which extend substantially end to end of the tubular body and are distributed to define a cleaning surface of at least 360 degrees; wherein the cleaning elements are retractable and extendable, wherein when retracted the cleaning elements are stored within the body until required for use; and when required for use the cleaning elements are extendable to an extended position in which they extend outwards from the outermost surface of the body; wherein the cleaning elements are held in a retracted position by a holding system, comprising one or more shear pins (24, 26, 28), until the cleaning elements are required for use and wherein an activation system is operable to at least prime the cleaning elements for movement from a retracted position to the extended position; the activation system comprising a setting sleeve, internal to the tubular body, and operable to move in an axial direction relative to the tubular body and operable to break or shear one or more of the said shear pins (24, 26, 28) such that the cleaning elements are at least partially primed for movement from the retracted to the extended position; wherein each cleaning element is biased to the extended position and wherein a shear pin prevents, until broken or sheared by a radial load generated by physical contact of the setting sleeve on the cleaning element, movement of the cleaning element from the retracted position to the extended position.