How to correct point cloud distortion
I've figured a difference approach to projecting the point cloud inside Houdini.

I had assumed that the 'Scene Depth World Units' would provide the depth of each pixel along a vector parallel with the camera vector, rather than angled to point at the camera as a single point.

So rather than projecting the point cloud as if the depth was correct, and then trying to adjust for the distortion this creates, it makes more sense to just project the points in the same way they were created.

So, knowing the position and rotation of the camera, I can then use the depth pass to project the points outwards from this location.