A blocked header pipe.

I thought i would share how i managed to unblock this without replacing the header tank.

If you connect a hose pipe up to the mains water the hold the end of the hose pipe in the header tank where the hole is for the small L pipe which goes into the main tank, this succesfully flushed the blockage for me. You will need two people to do this as someone will need to turn the hose pipe on when your ready.