Well, you're all being very kind and patient with me, so I've been back over the topic to see if I've misread something.
Firstly, bobthebender, of course you're right that water will find the lowest point, and that a holed cylinder coil will fill whichever cistern is lower/lowest, and you're right that I knew that. I didn't mean to imply that anyone was wrong about
that.
However, the OP is demonstrating a good understanding of the principles involved, and there's a consistent message from him/her that the water is flowing from the cylinder, back up its cold feed, into the CSS.
So, assuming that all of the obvious ball valve checking has been done, it all comes down to the relative heights of the cisterns; if the F&E (water level) is higher, then the cylinder coil is holed/split; if the CSS level is higher, then it can't be.
Given that we're waiting to hear which of these applies, and given that it's commonplace for both cisterns to sit on the same base, I was just playing my hunch card.
It's a risk that I'll be wrong, and you'll then all spank me with basin wrenches and spray my nads with expanding foam, but I was feeling mischievous so I'm taking that risk.