Welsh Water is one of the utilities I cover; the alkalinity and pH and chlorine levels of the water you receive are very tightly controlled - and they will happily give you their data for your area. They will certainly NOT be supplying at pH of 5.0; target range will be 7.0 to 9.0 max normally, free chlorine usually 0.2 to 0.5mg/l; alkalinity/pH might be artificially increased depending on your supply area typically using lime or caustic. Your mash could well be 5.0 however depending on what you make it with - but I bet the incoming tapwater is not. If it is, your test method is wrong - or you need to let DCWW know. I have never bothered to check start or final pH of my brews; not sure what it would tell me. NW Wales water is generally quite thin, low conductivity with little alkalinity/buffering.