If the smaller tank supplying the heating is overflowing and is not being caused by float valve failure you need to check the following:
1) Pipework may be blocked especially the cold feed to the cylinder. You can check this with a magnet. Remedy involves cutting out affected pipe and renewing or unblocking. See faq 24
2) The cylinder coil may have a hole in it and water is entering from the cylinder and exiting into the central heating system.
To check this, tie up the F & E ballcock, or turn off the mains supply to it, and see if it still overflows.
3) Check water level when cold. It should be 100 mm deep.
Remember, water expands when hot (hence the name for the tank) so a warm system will cause the tank level to rise anyway.