As is ever the case, I think you'll find its a little bit more complicated than that. HSG274 Part 2 benchmark is weekly flushing for low use outlets. If there are outlets in the retreat that are not being used regularly then they probably should be flushed to prevent stagnation. If they're in regular use then it seems unlikely that flushing is necessary. Typically, temperatures are taken at determined 'sentinel' outlets on a monthly basis and all outlets tested throughout the course of a year. Showerheads should typically be descaled and disinfected every quarter as standard. BUT All this is actually subject to a risk assessment by a competent person. There are a multitude of factors that might alter this status quo to do with type of hot water generation, any existing issues with biofilm, inability to determine sentinels etc.