my heater matrix was/is blocked (brrrrrr) with rad weld i guess (yuk)
anyhow i flushed n flushed n back flushed n front flushed the bstard to no avail...
so a tiny 2inch G-clamp on the bypass tube did the trick !
worked well, taken it off now (fear of blowing the matrix)
still got warm air, luurvely ... (beats de-icing the inside of the screen !!)