Thanks so much for your help Iain, im a big admirer of your work (who isn't!). Yes checked the albedo and all is above board but I might try stripping out the IES for this to see if they are the culprit. I have quite a few lights in the scene to highlight certain areas and also thought that maybe having more light in the scene would help clear up the GI. But perhaps its working against me. Going to take some env switching gymnastics as the outside lighting works really well with the HRDRI. I will try and strip everything back though. Sadly myself and the architect are at odds with the client on camera paths, with them favouring the literal tour of the building which leaves less room to hide unresolved stuff.
The C4D version has the 1.4 core but still missing some key features.
Thanks again, Rob