The heater matrix may well be clogged on the passenger side, the design isn't great and could just need a back flush. Another forum chap did this recently with favourable results (eventually). I intend to do mine at some point but can't be without at least some heat for the mrs, so I will wait until the weather improves.
If the heater matrix were leaking then you would be loosing coolant, is the fluid within your expansion tank consistent day/day, week/week?