A catchy song with lyrics something along the lines of "you think you've got it but you don't know" - does that ring any bells with anyone, please?
Sounds fairly recent.

I thought I heard it at the start of a Hollyoaks episode (on the 6th), but watching it on 4OD there was no trace of it, so I'm probably going mad or something

It's not Hey Ya by Outkast is it?

https://www.youtube.com/watch?v=PWgvGjAhvIw

By amazing coincidence, I accidentally hit the Radio 1 preset on the radio (I don't normally listen) and there it was, and the DJ kindly said what is was

Turns out it was Disciples - They Don't Know
https://www.youtube.com/watch?v=a5ITPHgfp1k