Fabia screen

Just returned from a holiday in Derbyshire & our 1'9 Fabia was great ! Only problem was keeping the windscreen clear. Driving back to the hotel I had to stop to wipe the moisture, then use the aircon & full fan to keep it clear. Not ideal, we were freezing ! The vent control was on 'screen' & there was no wet carpet or anything. Any ideas. Also, I keep the computer to current mpg setting, which is good at encouraging a light right foot, & normally shows we do well into the 60mpg, but this went haywire, fluctuating from zero to 112 etc. It's booked in for the dealer to check, but has this happened to anyone else ?