I went through all the apps I've installed in the past, and the only barcode scanner I've ever installed was this one:

https://play.google.com/store/apps/details?id=com.google.zxi...

So it might be another completely unrelated app that triggered the issue, or it might be this one, no idea!

I've reset my phone to factory and re-installed only as and when I needed an app and so far no more ads.