"dbroms" wrote in message
...
additional information

i have just watched the system and noted as soon as the demand for the
heating goes off , the pump seems to flow all the hot water into the
header
tank in the loft for about 5-10 seconds. The tank then seems to go back to
its correct level.

Is it when the valve is changeing state?

if so, what can i do?

Describe the system. Mid-position valve? pumps on the flow or return?
Cylinder? etc.