I'd be interested to hear if it was the heater matrix. I've had to change them on my last 2 Golf's and never had a problem with them. I went for the 'higher spec' £35 Valeo item too which I understood was OE.
Also, I thought it was commonly accepted that a passenger side leak could be a heater matrix and it's not normally the driverside? Dunno though, never been unlucky enough to find out.