Collagen Paste vs Mucosal Advancement Flap for FIA

Inclusion Criteria: * Clinical diagnosis of high cryptoglandular fistula-in-ano. * Patients must have undergone a prior EUA to characterise the nature of the fistula. * The fistula tract should be ≥ 2 cm in length. * Only a single internal fistula opening should be present at EUA, such that the fistula is suitable for treatment by insertion of a seton drain. * Patients must have been treated with a draining seton for a minimum period of 6 weeks prior to randomisation. * Patients must be aged ≥ 18 years and able to provide informed consent. * Fistulas must be of cryptoglandular aetiology. Exclusion Criteria: * Unable/unwilling to provide informed consent. * Contraindication to general anaesthesia or spinal anaesthesia. * Low trans-sphincteric fistulas. * Non-cryptoglandular fistulas (e.g. Crohn's disease, obstetric, irradiation, malignant, etc.). * Other perineal fistulas (e.g. rectovaginal fistulas, pouch-vaginal fistulas, etc.). * Complex disease in which more than one internal fistula opening is present and requiring concurrent insertion of more than one fistula plug. * Clinical evidence of active perianal sepsis. In the event that there is disagreement between clinical and radiological assessment of active sepsis/collection, the clinical opinion will prevail. * Cultural or religious objection to the use of pig tissue. * Absolute contraindication to magnetic resonance imaging (MRI) (e.g. cardiac pacemaker). * Patients with recurrent anal fistulas previously treated with a fistula plug.