A pressure relief valve has been suggested to prevent the problem I have of running out of hot water due to a punp being fitted to a bath shower mixer . Can the valve be fitted to the expansion pipe in the loft.

I don't think I understand your question completely... but, pressure relieve valve is usually installed somewhere on the cold water side of your plumbing system and discharge plumbed to the outside. Temperature and Pressure relieve valve is installed as close to a pressurized tank as possible ( this may be hot water heater ). Expansion tank is installed on cold water side, directly above hot water heater. Hope that answers your question. Back to you. Milo