Right - I'm coming back here to let everyone know that the stale CSS issue appears to be no fault of ReCache itself - mostly because after clearing redis on my local server again, the issue has since subsided, and I can no longer replicate it. ReCache has now been fired up again on my site, and it's seriously fast.
The monthly payment for this extension is extremely cheap when you consider the performance increase it presents. However, don't go using this on crappy hosting, as you won't see any benefit. I'm using a VPS with redis cache installed - both for WordPress (front-end of my website), and Flarum (the forums), and the speed is incredible. Have a look at this
Main Site
Lookup time: 0.004219
Connect time: 0.033650
SSL handshake time: 0.102745
Pre-Transfer time: 0.102862
Redirect time: 0.000000
Start transfer time: 0.694713
Total time: 0.748269
Discussion forums
Lookup time: 0.012339
Connect time: 0.019150
SSL handshake time: 0.042724
Pre-Transfer time: 0.042833
Redirect time: 0.000000
Start transfer time: 0.411799
Total time: 0.465647