Fan and heater controls on 2009 Avensis

[Dieseltread]

Hi,

No matter how fast or slow / carefully I turn the dials on the climate control unit, both for the temperature and the fan speed, the result is very random. For example the speed one will increase from 1, 2, 3, then shot up to maximum, I turn it down, get as far as 4 or so then it may skip to 0 or back to max. It took me twenty minutes yesterday to get the temperature to cross the halfway line into the hot zone after trying to turn it up a notch sent it plummeting to the far left (cold). 

Is there a fix for this or will it need a replacement control unit. If so how easy is it to replace the controller?

Image of controller type included.

Thank you in advance.

[FROSTYBALLS]

Moved to the Avensis club.