Right, so I've done my research, and as I already have the active sound unit under the car from having the M-Sport Plus 30D, I can purchase the add-on kit for the 50D that the below company is selling;
https://www.active-sound.eu/en/bmw/x...ive-sound-unit
Basically it gives you full app control over the active sound unit, over and above the default sounds that are produced by the sound unit from BMW, you can even specify the number of pops and mis-fires, plus the startup tone of the car.
I'll let you know what it's like once I've got my car back and fitted!