If the pop-up Window you are describing is the one I think you are describing. That Window pop-up first appeared months ago, long before this current pop-up issue surfaced. In my case, I closed it down (I can't remember now what button I pressed) and it never reappeared again. I could probably force it to re-appear if I go into my Firefox browser settings and delete all my Hornby stored cookies, but my system is stable at present.
Have you tried clicking the 'Cookie Icon' in the bottom right hand corner of the screen and deselecting the 'Marketing' option, then saving the changes .... see screen captures below from a Windows 10 PC. No guarantee that this will sort out your particular issue, but it might.