TL;DR: Issue was water pump, replaced it and is all fine.Thanks for the advice, and apologies for the slow response. Here was the situation and my solution, if it helps anyone.
I had tried the WD-40 method, but to no avail. Same whining noise.
Over the course of the next few months, the whining noise got progressively worse and worse, to the point where it was happening all the time (albeit between 500 - 2000 RPM).
Here is a video where you can really hear the noise: https://youtu.be/1x2xPaJ7B90This allowed me to get a long screwdriver and pinpoint, with relative precision, where the noise was coming from. I was sure that it was coming from one of the pulleys, specifically the water pump. I had also noticed that the coolant had been slowly leaking, this gave me some more evidence that it was this.
Took a small gamble (£25 part) and replaced the water pump. The noise is now completely gone!
Hope this helps anyone who has the same issue.