Re the leak; it could be the heater matrix. I only say this, as it happen to me in my old Peugeot 306. Not sure where the heater matrix is situated in a Mk5, or indeed if they're susceptible to this issue, but may be worth investigating. Seem to think radweld worked as a quick fix, but then the matrix needed replacing after it dumped its entire contents on my friend's feet going round a sharp corner.