I've a 2007 petrol focus. In last week i noticed the air coming out of the vents is warmer than usual. 

A week ago I did a 2 hour drive with temp on "Low", AC off and similar outside temp as now and all good / as expected. Just in the last few days I've noticed this is now warmer air - same settings, same outside air temp. For example now this is starting to fog up my windows, it's warmer air.

If I turn on AC it works as expected and I get cold air. So problem doesn't seem to be the AC part. Also checked coolant and seems fine, no sign of a leak, and no sign of engine over heating. Everything else on the heater console seems to be working fine like the power and splitting left side of car and right. It just seems to be this- with AC off heater not providing its usual cool air when set to Low.

Any thoughts on what could be causing this air to be warmer than normal when the AC is off - with none of these other symptoms?

Sorry this Doesn’t answer your question .. but .. why turn the AC button off at all? It’s best to run it all the time, This includes during winter when it’s set to blow warm air, so it de-mists inside the car quicker etc with the AC button on. 
Also running the AC all year round stops the seals from drying out and then leaking out the AC gas over time as well. 

From your description you've got the climate control rather than manual. The flap that mixes hot and cold air to control the temperature sounds like it's not shutting off the hot air completely. As to the reason for that it could be something lodged in it jamming it partly open or an electrical problem stopping the motor before it's fully closed. I've not delved into the complexities of the climate system so hopefully one of the resident gurus will be along soon with some better suggestions.