Thanks for the suggestion…. in fact the box came up correctly after further rebooting and without any changes on the browser side. For your information four different clients: Microsoft Edge and Google Chrome on the same instance of Windows 10, Safari on IOS and also on MacOS El Capitein. Behaviour was consistent across all platforms - generally the correct login screen followed by a completely blank browser window (with zero source HTML content) or the occasional variant of Server Error 500.
I did also confirm the first time around via TV menus that the box had already been updated and rebooted. I was trying to log in to change a 2.4 GHz wifi channel setting to reduce interference from external networks - despite the choice being set to automatic it had very cheerfully set the default channel to 6 on which at least two of my neighbours’ networks are also broadcasting.