Well, from what you have said there, it looks like it may just be that your Humax box has developed a fault.
It is more likely the software has become corrupt, or that it has got itself into a bad state, hence I expect to be back here again in a few months with the same problems once whatever gets the software into a bad state recurs.
I now have a new shiny box from Humax, they were extremely helpful. This box is working OK and seems to work OK from High ECO startup as well. I am still convinced my problems started with a software update but this probably happened due to timing issues with HDMI on the original trial hardware? HDMI now works, It seems (I hope) to be starting up from high ECO and I am watching Iplayer to see if it hangs.
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I have the same problem. Looks like I should ask for a replacement.
https://community.youview.com/youview/topics/humax_youview_hdmi_output_is_often_magenta_or_blank