Here's a shitheel thing that Spotify does, it secretly caches everything you 'stream' to a hidden folder buried away at:
C:\Users\%USERNAME%\AppData\Local\Spotify\Data
This had grown to nearly 10GB on my PC, which when you're running on a 119GB SSD as your system drive, is a fucking lot. (I'd noticed my SSD capacity going further and further into the red, and was wondering what was causing it, one Treesize later and I had my culprit.)
Note that it does this irrespective of where you set Spotify's offline cache to be, which I had already configured to be on a data drive, but the offline cache is not the same thing as its secret online cache. (You can understand conceptutally why they do it, since it saves you downloading the same songs every time you listen to them, reducing the load on Spotify's servers.)
Note I used the word 'downloading' there, as monitoring the behaviour of this folder I observed that as soon as you start 'streaming' a song, what it actually does is immediately download the song (in an encrypted format) to this folder, and plays it from there.
There is absolutely no way you can stop this behaviour, you can add a line to a buried away config file to set the data folder to a max size in megabytes, at which point Spotify will start to 'churn' the data in this folder as required, but fundamentally Spotify won't work unless that folder is there at its default path and it can write to it.
Anyway, MKLINK seemed to offer a handy solution with its ability to create directory junctions, and I can confirm it works perfectly. If you ever come across the same problem, then a command such as this (modified for your username and preferred new location) will do the job. (Run from an elevated command prompt, make sure Spotify is closed (not minimised) and delete the actual data folder before running the command as MKLINK creates the junction when run.)
Code:
mklink /J "C:\Users\Alex\AppData\Local\Spotify\Data" "G:\SPOTIFYSECRET"
(Note that after a couple of hours of listening to music after I'd made the change, the SPOTIFYSECRET folder had already grown to 545MB, so it's quite a greedy bastard.)