Thanks for the reply.
Unfortunately, looking around on YT, it would seem that it's a wide-spread issue, particularly when trying to get 10-bit working, though in my case it's 8-bit.
Looks like I'll need to buy the HEVC codec from Microsoft (which is peanuts), but I was just curious why it wouldn't accept the video track from the footage.
Alternatively, I might try Shutter Encoder to transcode, then I can drop in the footage and apparently, that will work.
At first, I thought it might be something to do with the colour space/format, which I know almost nothing about.
In OBS, I'm using NV12 (4:2:0) and I did try a variety of other options, but to no avail.