If you use Chrome as your mobile browser then all you need to do is open the site, click on the Chrome settings button (the 3 dots top right) then check the "Desktop site" box. It will only affect that tab in the browser.
Done

That's a similar method to the browser on my Samsung phone (I think it's based on Chrome?), but it only persists as long as I keep that tab open. When I close the tab and/or browser and start again it resets to mobile version.

So no way to re-introduce the desktop button (or some other way in profile settings etc) to retain my desktop preference each time, i.e. as it used to do?

Apart from this annoyance the new look is good. I presume the 800 forum will follow suit?