Many thanks for all your replies!
I am currently training short-field landings and takeoffs in my local airport, as later on this month I'll be going for a navigation flight to an airfield with a 680m long runway. Does anyone recommend 40 flap? Or will it be ok with 20?
I'll be going up again locally for some more training and will let you know how it went. I'm sure I'll be fine after reading the very valid comments.