Manual air conditioning temperature fault

hi all,

a bit of a funny one this - I have a 2012 Octavia estate with the manual air conditioning.  The temperature dial on the heating control has started to behave oddly.  If I turn it to 3/4 hot, there is heat from the vents. But if I turn it all the way hot, the temperature goes tepid again!  If I turn it full cold it works, it just seems like the last, hot bit of the dial isn't working properly.  

Is this likely to be an issue with the dial, or the temperature control motor?  Is there any way to tell apart from switching out the dial?