Abstract: A rotary cutter head assembly comprises a cutting element including a substantially spherical enlarged retaining portion, a filamentary flexible elongate cutting portion, and a substantially cylindrical intermediate strengthening portion, which is integral with and between the retaining portion and the filamentary portion and is of smaller cross-sectional area than that of the retaining portion, and a head including a first flange which is substantially planar, a second flange which is substantially planar and which extends substantially parallel to and is spaced from the first flange so as to define a channel between the first and second flanges, in which channel the retaining portion of the cutting element is releasably and pivotally retain.