Okay, so I think my D810 is dead. Was reviewing some images on it yesterday and it froze mid-chimp, pulling the memory card had no effect and it wouldn't switch off either, the image I'd been looking at remained on the screen, in the end I had to pull the battery.
Putting the battery back in resulted in 'Err' flashing in the LCD and it doesn't recognise any memory cards. Have tried a two-button reset to no effect. It's going back to Nikon on Monday but it really couldn't have come at a worse time.