Unfortunately many plumbers and gas fitters don't often know much about these types of system as they are usually found in bulk lots in newbuilds from the last 20years. So they don't get to see them unless an estate nearby is fitted with them. Problem is, if it is the coil then its a big expense to replace it. I would want to make absolutely sure though. As someone mentioned before the best bet would be tying up the ball valve for a few hours, but then it sounds as if your struggling! If you post your location, maybe someone from the forum can offer their services.
