Overflowing HOT WATER feed tank

[god55]

NOTE:- This is not the Boiler/central heating header tank.

Hi,
I have a gas gravity fed hot water and central heating system with a freestanding boiler in the garage. The system has an indirect hot water (HW) cylinder with a standard pumped central heating (CH) system. The
house (HW) circuit has its own dedicated 25 gallon hot water feed/expansion (Header) tank which is in the loft under which is located separate 4 gallon dedicated CH/Boiler feed/expansion (Header) tank, the HW header tank is approx 60 cm above the boiler header tank, both have their own separate mains water supply, feed and expansion pipe work. This system has been untouched and working perfectly for the last twenty or more years (With minor maintenance and normal servicing). Three weeks ago, I noticed that the HW header tank was overflowing, I replaced the ball cock washer but next morning it still overflowed. I replaced the ball cock completely but unfortunately, it still overflowed. I turned off the water supply to the ball cock, however next morning the tank was full and still overflowing. Since then I have carried out a series of tests to establish where the excess water is coming from.
1) Turned off the boiler (Via the central heating programmer, pilot light still running) - STILL OVERFLOWING.
2) Turned off the boiler plus turned of the mains water supply - STILL OVERFLOWING.
3) Turned off the boiler plus turned of the mains water supply plus hung a container under the HW (Cylinder) expansion pipe – HW (Cylinder) expansion pipe CONTAINER DRY - STILL OVERFLOWING.
It would seem that the only place that the water can be coming from is up the HW (Cylinder) gravity feed pipe?
Tonight I will repeat (3) above plus turn off the HW (Cylinder) feed pipe supply, this should result in no additional water entering the HW header tank.
All the while that this problem has existed the house CH (CH programmer) has been off. At all times the water level in the CH/Boiler feed/expansion tank has remained completely normal.
It should be noted that this system is a very basic with no diverter, 3-way, or zone valves in the system.
In addition, no immersion heater was switched on during any of these tests (I unplugged this at the start to be sure).
Question.
If I confirm that the excess water is coming into the HW header tank via the HW (Cylinder) gravity feed pipe has anyone any idea what may be causing this problem?
Initially I thought that this may be due to a failed indirect coil in the cylinder. However I cannot see how this could be the case when the water is able to push uphill into the header tank, some six feet above it, particularly knowing that this happens when the boiler is COLD, with no pressure in the circuit caused by heating.
I am baffled; any suggestions would be greatly appreciated.

Garry