How old are they? How many? I just started litter training with my two eleven-week-old boys (you can probably see my post about it below yours under the same heading), and I'm learning very quickly that it isn't really training like I would do with my dog so much as it is leaving lots of clues, sitting back, and hoping they figure it out on their own.
I'm not really sure what "litter training pebbles" are, or what kind of smell they have, but ideally whatever you're using shouldn't have any type of smell or dust, since rats are very sensitive to that kind of thing. If they have Yesterday's News or something similar in England, I would suggest that. But make sure you get the unscented bag; I have a scented bag for my cat, and there is a big difference.
I use a plastic triangle-shaped litter box that snaps into the corner so they can't move it, but I know a lot of people here just use a small plastic container. If you're having issues with your guys tipping it over, I'd punch a few holes in the side/s against the cage bars, and zip-tie it to the bars. If your guys have access to their cage during outtime, they should go back there to use their litter box, but if not, you can just get a second box.
My boys are doing that as well at this point, the magically missing the litter box and pooping all around the outside thing. Give it time. Keep picking up the poop and putting it in the litterbox. As Jorats explained to me in my thread, they don't usually become reliable at their litter box until around six months old, so if you have babies like me, you just have to wait, or if you have oldies, they might not have 100% control of their bowels anymore, so it might be more difficult.