Lumpy tickover vrs tfsi

Hello everyone, need some help please.When i get in my car (octavia vrs tfsi 2010 estate) on a mild morning,its a very lumpy drive at low revs and tickover,until it warms up.On a cold morning,there is none of this lumpiness,and she drives nice and smoothly. I was thinking maybe a sensor of some type? Also, what sort of mpg are people getting on theirs ? I'm getting 35-36 if on a run,but around town it seems like the fuel gauge is dropping before my eyes.Recent change from diesel to petrol,and am unsure if this is normal.Many thanks.