Solved
I found a kind Burley trailer owner who had a look so the answers for future searchers…
The strap goes under the seat at the front.
If you soak the strap and seat in water it will expand enough to fit it easily. I soaked for a day and half but it probably doesn’t take that long.
You will probably need to take the suspension off to get to the bolts for the seat back. You will want to attach the springs to it’s mounts after the back mounts are fitted but before the front.