Hi, Last couple of days our bathroom hot tap has been misbehaving. When turned on the hot tap emits water under normal pressure but once it's been running for 10/15 secs the pressure gradually drops to about half of what is normal. I thought it may be a problem with the cartridge so I switched the hot and cold and nothing changed, the cold is fine but the hot not. We are on a gravity fed system and all other hot taps in the property are running normally. The water is still really hot but this fluctuating pressure is a bit weird - any ideas ?