My current NUC came from QuietPC.com - you can custom-spec them quite freely and I've been very happy with mine. Saying that, its main use is as a 24/7 torrent machine - I got the slightly taller one so it has an SSD for the OS and a 2TB 2.5" hard drive for all Jim's files. I haven't tried using one as a media centre. My media centre is a Raspberry Pi running Kodi that lives next to the TV and just streams all the videos over the network from my PC.

Still, they custom-build them for you, they come with a guarantee and you can choose the specs to your heart's content. They also do AMD ones which might be an idea as you get (admittedly basic) Radeon graphics with those, so might be better at handling the odd game than Intel's built in graphics.

Thanks Dave!! I'll have a look... 

Your way is less of a nail-biter for sure. One other possiblity is refurb SFF office pcs, though considerably larger than NUC/set-top box, you can get a pretty powerful rig for less than half what you'd pay for the latest model @ retail. Also they generally have Win10 Pro license on board.

I've been doing just that, our place went through a cull on desktop machines and replaced them with laptops so I've been able to liberate 4 SFF HP boxes from the great scrap pile in the sky with i5s and at least 8GB RAM TBH in each. 

Found a nice bonus in one, a spare 1TB HDD.

And dust. So much dust. 

Those old HP boxes never say die! Dirt cheap, tough as nails, and surprisingly powerful considering the first part.

Ended up getting a PN51 direct from Asus 32gb RAM, 1Tb SSD.

Should keep me quiet for a while.

Installed Forza Horizon 4 Ultimate for a giggle, actually runs fine, admittedly not in full fat settings but I can't tell the difference in graphics nor performance against my Xbox Series X, which is nice.

:)

Looks very nice. I built a tiny PC to dump all my music CDs onto 42301.1 based around an ASRock STX motherboard. I imagine it's about the same size and, in the end, probably cost me almost as much as the much-more-powerful thing you've got. It was fun, and it did have space for the 2 SATA drives I wanted (along with an NVME drive), but I'd probably have been better going for something ready-made.

Looks a nice little system, hope it does the trick! Back when I bought my NUC, Ryzen systems were pretty much an impossibility to find unfortunately, still for a torrent machine, it does pretty well.

So far - so good (yes)

On the back of this, I'm upscaling my Alienware with a 1Tb SSD in place of it's current 1Tb HDD.

Presumably I can just take out the HDD, insert SSD and then install Windows using the USB thingy that I got from Dell for clean/re-installing/repairing Windows 10?

Yep, should work fine. And of course if anything goes wrong, you can always pop the HDD back in temporarily to resurrect the system.