Hi everyone
Thanks for the replies.
I can confirm it's a BME280 sensor as its also got a humidity sensor.
@WVMarie, I see where you're coming from, I'm using an external 9V battery to power the Arduino whilst the drone is in flight, not exactly wanting to tamper with the electronics of the drone itself as it would void my warranty etc.
@Robin2, I've currently got the Arduino measuring pressure changes every 0.75 seconds so I'm going to experiment with that and see if that gives me enough time for the Arduino to detect a 'fall'.
@AWOL - Thanks for the correction, air pressure does rise when altitude is dropping.