Its ok. It worked in the end. Im not sure what I did. I know I reset the settings on the dialectric material. It was possibly to do with the light_linking. The scene has image-based lighting which I unconnected from the windows, I could see alot of it in the reflection so I turned it off thinking it was overpowering the image behind. But when i reset the material and reconnected the IBL it worked. A bit of a 'doh', moment