The coolant circuit on Wapiti is used to heat hot water, support a space heater in the main saloon as well as cool the engine, the circuit is quite large and includes a storage tank (see pic). A 240v element in the calorifier and a seperate diesel heater in the aft lazarette is connected to the circuit as well for alternative heating sources, so a bit of stuff going on in this circuit.

When I top up this engine coolant tank, over time, when running the engine it drains into the plastic overflow tank connected to the heat exchanger/radiator cap on the engine. No problems with running temps etc it just ends up filling the overflow tank and then spills into the engine room bilge.

Not sure why, is it a design fault, should I close off the heating circuit ( via ball valves on top of engine) if not needed to heat water, or just stop topping up the storage tank. There is a valve on the engine coolant tank lid, should that be open or closed?.

cam my boat use to do the same thing ,i traced it to the water heater which had a bad solder joint in the fresh water side. slow release of freshwater in to the coolant and the radiator cap would bleed it back to header tank and into bilge. to find it turn of your water pump and bleed out fresh water pressure and reset header tank to low level take for a run it should not go above the high level when hot.
my tank was a volvo heat tank they solder copper to stainless pipes

Yes had the same problem. The header tank would fill up and overflow. Started slow and got worse. As grich62 said it turned out to be a leak in the hot water heater. Had to throw it away and get a new one as it was in the unit somewhere.

Thanks for the tips I wasn't thinking along those lines at all, but now that you mention a slow leak in the fresh water side I do recall the fresh fresh water pump going off randomly from time to time so there must be a slow leak somewhere in the system.