Bought a new tube of filler today, a slightly different type but the same brand. It suffered the same problem: it sticks poorly to the card but well to the ice lolly stick. But again, a little water on said stick solved that well enough, so I now have all four corners landscaped:
That done, I could fit the rails:
The set gives you four rails, but you only need about one and half each side. You get fishplates to connect one side, while the other is meant to splice onto a second section of rail. I wanted the fishplates visible, so I cut a short piece off one rail and attached it to the next with those plates, then added the remaining section to the other side and cut it to length. That done with two, I glued them to the sleepers and added the little retaining plates-with-bolts on the inside. All of those only on the three sleepers where they will be visible, as you can see in the photo above.
With the rails in, I could fit the actual crossing:
Once those were in, but after taking this picture, I used a little more filler to close the gaps between the beams and the paving. Now I need to wait for it all to dry so I can start painting.