Have same problem. British Gas, local plumber etc tried to assess. My guess is that the "calorifier (twirly piece of pipe in the cylinder if you have one) is furred up.
What I haven't the nerve to do yet particularly over the holidays is to put a defurring material in the cold water tank a run hot water taps for a little while to let the materials get into the cylinder. Let it do its job and flush through. The trouble is flushing through may cause joints to leak and you may need to drain whole system down.
I am going to get a plumber to replace the cylinder with a new and take opportunity to have a properly insulated one.
You may not have cylinder and may be straight off the mains with a combi type boiler in which case, may be furring around the combi calorifier but am getting out of my depth here.
May be worth thinking about and logically seems right to me. See what the instructions are on the defurring material.
Frank Langan, December 2007
