Heater matrix

[gazza1970]

Morning all ,
its pick your brains time ,
Driving home in the early hours in my b5.5 sport and I turned the heating up from low to 22c I then got a strong smell of antifreeze through the vents , turned it down to low or up to hi and the smell eventually goes ,
Temp doesn't move off 90c , it's not using any water and it's had a new rad , so I'm now thinking it could be the heater matrix weeping,

So you now what my next question is
How big of a job is it