Joe could be right, we all see different types of problems. My experience on this is that it usually comes down to a new push button control panel, assuming that this is the same control panel as a US model. Joe has already asked if it has the auto climate control or not. The solder joints can crack and cause problems liek you describe. You are right though, usually you don't get hot air out the center vent, on that car I think it's only supposed to be fresh or A/C cold air. But this push button panel can screw it up that way. That's why I'm leaning more towards the push button panel than a sensor. In a perfect world the system would be tested using a break-out box to check the sensor readings. You can also test for a bad solder joint by running the system with the push button panel removed with the connectors hooked up of course, and kind of twisting the controller slightly to see if the system goes into a failed state. I recommend only a new push button panel, I see the rebuilt ones failing quite often within a year or so (if this proves to be the problem).
ps, Problems with the vacuum actuators and the vacuum control block are fairly rare on this, plus you also have the air temp problem going on at the same time. Gut reaction is that it's a controller fault, but the actuators and control block can be tested as well, not what most DIY'ers can normally do without borrowing special tools (breakout boxes and adapters, etc)