Which parameter can I change to add in a bit of up elevator to aileron movement as my plane drops a little when turning.
You can try PTCH2SRV_RLL (plane.ardupilot.com/wiki/ardupla … ch2srv_rll) but this problem is often cured by firmer pitch tuning - ie, increase your Pitch P value or redo autotune with more aggressiveness.
Or (depending on your transmitter) add a mix to the aileron channel which includes a percentage of up elevator.