I can't remember exactly what or where the signs are. But I've been up that way for Albion games three or four times since. The only time I forgot, I stopped at Clackett Lane Services to sort it out on my phone. Though it turned out I had the rest of that day to pay anyway, even after crossing.
It's been well publicised. There are signs all the way up to Dartford in both directions. There has been at least one thread on NSC about it. You can pay up to midnight after you've been through the (former) toll booths.
When you drove up there, did you not think it the slightest bit strange...