SOLVED: Annoying pop up advert icon - what's the source?

It's a small circle with an image in it, half on my Samsung galaxy S5 mini screen, half off. I click it to see what it is and it takes me to some 'jackpot' utter sh1te. This gets rid of the icon, but it's back within an hour. It's making my (already slow) phone run at a snail's pace. Avast doesn't detect it as malware, so any ideas? I'd seriously like to stamp on the balls of the numbnut f*ckwits that think this sort of marketing actually works. (See the attached images - icon is just below Facebook).