Only a guess IF your hot water tank is hot then it may be an airlock. connect hot and cold taps with a hose and force cold up the pipes.
If the tank is cold then it could be anything so afraid it's a Corgi registered plumber you will require who will be more familiar with this system.
However are you in a bungalow with the hot water tank on groundfloor and not in loft? if so it may be the circulation pump has packed up or is blocked with silt and the water can't get past or does so slow iwhich means a replacement pump. Usually a Grundfoss. They are easy enough to change with the cut off valves each side of the pump. if you do change it then note down the terminal wire positions in the pump. Turn off the boiler shut the two valves Two wreches and undo the pump nuts note which way around the pump goes. Grundfoss pumps around £38 a guess. Good luck if you do it...... If the water has a strong flow forget the pump call an engineer Corgi registered a central heating engineer would as the plumber said, be more familiar with the system and know what the fault is before seeing the system . bob
Bob
October 2005