HOT WATER PROBLEM

[K4Gas]

Mate text me this earlier:
When Im filling the bath up the hot water stops all together after a few inches in depth. I googled it and it sounded like an air lock. So I sent cold water through the pipes while leaving the hot tap open. This seemed to work at first and hot water started again. Its happening every day now, I have to send the cold through the taps again for a couple of minutes to get more hot water.
Its not a combi boiler, I have a water tank.
Any ideas to help

 

[TCCHeating]

Check the water level in the cold water storage tank in the loft. It sounds like the ball valve is either stuck on something (causing a very low level of water in the tank). or the tank is re-filling very very slowly!