Cheers for that useful info - I had a very quick look at them last night, but not properly. I'll make sure to have a good look around those areas, as it is definitely a possibility!
I did notice that where the wheel arch comes down to the bottom at the leading edge, it comes a bit further down than the bottom of the car's body. Just infront of that, there's a large hole where some sort of joint / nut is - I guess wind could be getting in up there...
I'll check again.
Further to the above, I checked the wheel arches again at lunch.
The rear arch on the passenger side is as expected, fitted well and made of one solid, smooth piece of plastic.
The drivers side however, despite being well fitted and no loose parts, is different (but looks original). There's a large opening right at the back, with what look like pipes running down - my guess is the fuel filler pipe?
Anyway, this large hole is enough to potentially make a noise, as any side winds going into the wheel arch (which would be high pressure anyway) would be forced into this opening at an even higher pressure.
I think I'll take some time after my weekend away to remove the wheel, and then fully clean and dry the wheel arch liner. Then I'll cover up the hole with some strong black duct tape to see if it solves it...
Again, thanks for this suggestion - wouldn't have considered this without your ideas!