XC60 heater stuck on, like a Swedish Sauna inside!

[jptsdl]

The heater on my 2011 XC60 T6 is always on, once the engine has warmed up, with the AC off...

The details:
- model with dual climate control with temperature selector on LHS and RHS
- if temperature set to minimum (15degC), air temperature at both LH and RH vents is 38-45 degC
- if temperature is set to 30degC, air temperature at vents is the same; 38-45degC (no change)
- if temperature is set to MAX, air temp at vents climbs quickly to 70degC (!)
- outside air temp when testing was 18degC
- both sides have the same temperature

The AC works fine but has to work overtime to overcome the heater (AC compressor never cycles off [as it should], as it struggles to maintain reasonably cool temperatures).

What could be causing this temperature malfunction?

The Volvo dealer here in Lima cannot solve the problem ("nothing shows up on out computer so we don't know what to do"!). I am getting desperate here - the car has turned into a real Swedish Sauna! I just need some hot rocks and a bucket of water.....