Peugeot 206 intermittent AC problem

[KMud]

Either my google-fu is weak or this is a bit of an odd one...the AC on my 206 (boggo y-reg GTi) works intermittently. The AC only blows intermittently, and the fan-speed adjusts quite frequently when it does blow. When it blows the air is cold, so I think the actual refridgeration loop is fine. I'm thinking maybe the cabin temperature sensor is borked, or perhaps simply the fan. Any advice on what to do first for troubleshooting (i.e., minimum fuss and cost)?