Hi indeed very strange, the only other thing may be the HDMI lead do you have the right specification it should be a 4k high speed one, even if you don't have a 4K TV, or try a different port on your TV, hope you solve it. Regards Micky
So i think this is the final update on this as the system has cured itself. I didn't do anything different, just turned the box on and it was in colour after two days of being black and white.
The only thing that appears different is the GUI, so it appears that there has been a system update. I don't know why that would have this effect but the fact it happened at the same time could be a coincidence but I would guess that seems likely to be the cause.