Un-blinding: Should a serious, adverse event or clinical situation occur that required a participant to be un-blinded, that decision will be made by the Chief Investigator (CI) and reported to the sponsor as per standard R&D Dept. Governance protocol? Time scale: 12 months. In the short term the study will provide preliminary data to on the impact of administering a PNB to women undergoing routine posterior vaginal wall surgery. In the longer term this data will inform a future definitive study into the clinical and economic impact of the routine use of PNB in this context. Results will be written up and submitted for publication in a peer-reviewed journal.