A fastener comprising: an elongate band having a first end region and a second end region, the band having an inwardly facing surface and an outwardly facing surface extending between the first and second end regions (3,4), the inwardly facing surface and outwardly facing surface defining respective pluralities of teeth thereon, such teeth being located at least in the vicinities of the first and second end regions (3,4) of the band; and a holding member for holding the band in a closed loop, in which a first end of the holding member provides a first opening for receiving the first end region of the band, and in which a second end of the holding member provides a second opening for receiving the second end region of the band, and in which the first and second openings (11,13) are in communication with a chamber for receiving the first and second end regions (3,4) of the band, and in which the holding member comprises an inner surface configured to be directed towards the inside of the loop when formed and an outer surface opposed to the inner surface, the outer surface having at least one access opening providing access to the chamber, characterised in that the holding member further comprises at least one locking mechanism arranged to be received within and moveable through the at least one access opening, the or each locking mechanism being moveable between an engaged position in which the locking mechanism concerned is arranged such that at least some of the teeth on the inwardly facing surface engage with at least some of the teeth on the outwardly facing surface of the band, and a disengaged position in which the teeth on the inwardly facing surface are disengaged from the teeth on the outwardly facing surface of the band. an elongate band having a first end region and a second end region, the band having an inwardly facing surface and an outwardly facing surface extending between the first and second end regions (3,4), the inwardly facing surface and outwardly facing surface defining respective pluralities of teeth thereon, such teeth being located at least in the vicinities of the first and second end regions (3,4) of the band; and a holding member for holding the band in a closed loop, in which a first end of the holding member provides a first opening for receiving the first end region of the band, and in which a second end of the holding member provides a second opening for receiving the second end region of the band, and in which the first and second openings (11,13) are in communication with a chamber for receiving the first and second end regions (3,4) of the band, and in which the holding member comprises an inner surface configured to be directed towards the inside of the loop when formed and an outer surface opposed to the inner surface, the outer surface having at least one access opening providing access to the chamber, characterised in that the holding member further comprises at least one locking mechanism arranged to be received within and moveable through the at least one access opening, the or each locking mechanism being moveable between an engaged position in which the locking mechanism concerned is arranged such that at least some of the teeth on the inwardly facing surface engage with at least some of the teeth on the outwardly facing surface of the band, and a disengaged position in which the teeth on the inwardly facing surface are disengaged from the teeth on the outwardly facing surface of the band.