As well as being a filler pipe, you can just see a wooden bung in the end, which is resting on the roof line, It also serves as an expansion tank. However the CC is very effective and the water hardly expands at all so the level hardly moves.
Now that the expansion volume is known, hardly any, the filler pipe has been shortened to about 15 inches, just above the black tape marker, which serves as my fill point.
