Thanks for the replies! To be double-sure what the fan was actually doing, I headed off-road again. This time, I went with a mate with a motocross background. Long story short, I realised just how slow I was going previously, and added an extra 10mph to my speed at most points which also put me up a gear. Voila! No fan coming on! I guess the fan coming on must be an audible newbie warning for other riders!

P.S. My mate fell off and I didn't. The moral of this post is to go fast enough that your fan isn't on, but not so fast as to fly off your bike!