All Mod Cons

Any more thoughts on this anyone?
My short term solution, though not ideal, was the following...

When the system was completely cold, I removed quite a lot of water from the header tank to take the water level to the point just before the ballcock cut in and started filling the tank up again. I then tied up the ballcock, so no water could enter the tank.
I marked the water level (with the system at cold), and then turned on heating and every hour or so marked the level in the header tank as it rose.
After a while I turned the heating up to full and left it for a while before marking the water level for the final time. At this point the the level was about an inch under the overflow pipe level, but quite a lot higher than the mark when the sytem was cold.

I've now turned the heating off and will monitor the level as it drops down to (hopefully) the first level mark.

As the ballcock is tied it can't let anymore water in if the level falls below the 'cold' level. I'm guessing this is what must have been happening. Then, once the heating warmed up the water and the level rose, we got water coming out of the overflow.

This is not a great solution but will do until I find the proper answer to this pronblem. It saves wasting a load of water out the overflow twice daily as the heating goes on and off.
If anyone has anymore ideas I'd be grateful to hear them.
Richard,.