Lost most of my expansion tank's coolant. I thought I had found why - water pump - but opinions here differ and I'm confused.
If the pump is leaking then coolant will dribble out of the drain hole on the underside of the pump body. You can't see the drain hole when the pump pulley is in place - you need to undo 3no 6mm bolts to get the pulley off to check.
2 possible sources of coolant dribbling down the front of the engine are the aluminium plate on the oil cooler (they corrode if the coolant isn't changed periodically) or the thermostat body / transfer tube (again they corrode if the coolant loses it's anti corrosion properties due to being too old).
If you can definitely see where the coolant is leaking then you have found your problem and are well on the way to fixing it
If you are not certain then I strongly recommend that you make/get yourself a test cap for the coolant tank so that you can pressurise the system when the engine is cold. It is so much easier to find a leak (or leaks) when the engine is cold. (and you need to have the intake plenum off to be able to see the oil cooler plate and thermostat...............)