I'll do I spose as the others aint shown up.
Couple of things if you cant see a leak. Either the heater matrix is leaking (steam/smell from heater) or its burning it (steam from exhaust).
Really you need to pressure test it and have a good look round with a torch, if that shows nuffin then the next step will be a block test, this will tell you if coolant is getting into the cylinders.
L series engines rairly suffer with head gasket problems so I reckon you have an external leak somewhere.