I reset my network connection, but it made no difference.
Well I had a little play with my two boxes just now. I found a little nugget of info on accessing the Roku internal signal strength page. Home button X5 then up, down, up, down, up, via the remote.
My suspected dodgy Roku Express indicated a signal strength of around 15 (I assume out of 100) Simply switching boxes to my Roku Premier, and touching nothing else, I get an indicated 34. So more than double.
So pretty much 50% less sensitivity between the two boxes. Whether it's been accidentally hobbled by an erroneous software update or its just packing in, who can say.
Decided to bite the bullet and order one of the newer Roku streaming sticks as they are supposed to have 4X the range, so should fix it. Even though I'll be a few quid out of pocket, yet again..
Unless anyone has any suggestions for a better device for around the same money??