I have a gravity fed hot water system. Until recently it had a healthy system presssure. No changes have been made to the system and no recent work has been carried out. Pressure has dropped off to the point where it can take 20 minutes to run a bath. There has been a corresponding drop (although not quite as significant) in cold water presure. Pressure starts good and tails off as if a down pipe were emptying but not being backed up.

All valves feeding the header tanks seem to be open and depressing the ballcocks results in appropriate water flow into the tanks.