I'd be interested to know how you are so sure

If its a fake then someone went to an awful amount of trouble even to the point of getting the 1940's UK and Australian / NZ accents. Did you spot the commonwealth crew member ? You'd have to be British to notice the very small twang in one of the voices.
Realistically if this is not the real deal someone spent quite some time, professional skill and indeed money on this. Apart from for a film i can't see why someone would want to. I've not heard this clip in any film and i've seen all that i know of.
I do also know that audio recording devices were carried as routine by ELINT aircraft of 100 group and on occasion by main force aircraft. So its entirely plausable.
IMHO

edit ***
The thing that still grabs me afte listening to it many times is how cool and professional they are. More often than not meeting a night fighter resulted in death for the bomber crew or the rest of the war in a cage at best. They came within a nats whisker but 10 seconds later its "Shows over chaps on with the job"