Phil i know you have all the best intensions but i just wouldn`t bother mate,if they happen to be in and there normally out when i clean if they say anything i will explain but other than that i just carry on.All this giving letters out business just encourages people to inspect every pane of glass in my experience,by just turning up and getting on with the job WFP will show the customer that you have confidence in the system,start trying to explain it and you`ll dig yourself a hole.I know it`s not easy but treat them like all the other WFP customers,i still trad a few but only the really good earners where WFP is not needed.