The expansion tank on my wedge,whether 2.0L or V8 wont run with the header / expansion tank full to the top. It runs at a level about 35 to 50mm below the filler cap. If I top it up cold, drive for 30 minutes or more, then leave it to cool down, the level stabilises at 35 to 50mm Below the cap again. Could that be the missing 200ml you're looking for?
I've had a head gasket leak coolant in to a cylinder on a different vehicle. Lots of white smoke (steam) blew out of the exhaust pipe, bubbles from the cylinder blew up from the header tank which tried to drive out the coolant with the cap off.
I did have a leaking water pump on my 2.0L engine, via that nice hidden slot underneath the carbs IIRC.
Hope this helps,