Those settings are ridiculously high. It looks more like a geo issue. What happens when you hide the front geo? Is it still there? Could it be coincidental geometry? Do you have Subdivision Iterations on any of them?
Hi Lee. Thanks for reaching out. I'll double check that! It looked okay originally.
The renders will be used in hi res print. What settings would you recommend, whilst avoiding grain.
That seems to have been the issue @Lee Griggs
How embarrassing! Thanks for that.
Glad you fixed it. All of the 4's concern me. Especially for Diffuse Ray Depth. These will be very expensive to render. For a scene like this, I imagine 4 light samples would be enough and maybe you can drop the others.
What happens when you use the default settings with Camera (AA): 5 and 3 or 4 light samples?
Thanks Lee. I've tried some lower settings. Halved the render time, but I have some black areas in the transparency.
Increasing Transmission Ray Depth should eliminate the black areas. Tutorial example here.