Help! - Octane + Backplate compositing problem

[e.mike.h]

I'm a relatively new octane user and have come across this issue for quite a while now. When I'm trying to composite a 3D object onto a backplate I get this black shadow wherever the plane is that I am using as a floor. Weirdly it only seems to happen with car models. Yes I have checked the matte option in the material settings and I am using a HDRI for the lighting. Thanks in advance.

[niestudio]

You shouldn't need the plane from Octane if you already have the backplate in photohsop. Just the shadow pass.
Are you using the render layer system?

[e.mike.h]

I wasn't using that system no. However when I do use render passes the shadow layer still has the same issue. Not sure how to keep the shadows without a plane involved.

[niestudio]

You do need the plane to catch the shadow, just not the one with the backplate texture on it.
What format are you rendering to?
Can you adjust the levels on that layer to make the light grey white so you can composite just the shadow?

This video is in Octane V2, so the buttons are different, but the techniques remain basically the same
https://vimeo.com/119848103