Emptied loft tank to clear hard water and change faulty valve, after filling tank hot water to all taps runs slow then stopes

Sounds like you've disturb some minerals built up in the pipe walls that is now blocking the inlet ports are your faucets cartridge. The solution would be to shut the water off to the tap, open it up, and remove the cartridge. Check a clean the inlet ports. Before you reassemble turn the water on for a moment to flush out the supplies. Reassemble and test. Good luck, Tom