I may have missed something with Q5.xx but I can't find a way to produce a stand-alone runnable game as in Q4. It seems only possible to play the game in the editor, which is why I've switched back to Q4.15.

Yes there's no stand-alone EXE maker for Quest 5. I haven't bothered to implement one as frankly Windows-only desktop applications are on their way out - most people play Quest games via their web browser these days, and Quest 5 very much makes that the focus with hyperlinks, embeddable videos etc.

Is it possible then to create the html files and upload to my own webspace to play online or only through textadventures.co.uk?

You can upload to textadventures.co.uk and then embed the player in your webpage using an IFRAME like this:

<iframe src="http://play.textadventures.co.uk/v5/Play.aspx?id=55" width="1000px" height="600px"></iframe>

Thanks Alex, most helpful.

It's also possible to install the webplayer onto your own server, but it requires a windows server with ASP.NET which is fairly uncommon with most shared hosting providers. But I just thought I'd mention it.