Well, this is weird...I've checked and I installed Windows 10 on the 11th of October, so it's not the 31-day thing that's stopping me from getting this update. I definitely don't have it installed, I checked the build number as above, so I'm not sure why it's not available for me.
The one oddity I can think of is that I distinctly remember having the option to defer upgrades, which I thought was odd as I have the Home edition which I thought didn't have this option. I've read some info about some update that may have added this option to the Home edition, not sure if that actually happened though as there's a lot of false info about Windows 10 out there. Anyway, I did have this option, and now I don't, so I wonder if me ticking this option before has left it "set" somewhere in the registry...can't find info on this either from Googling. I haven't spent all that much time on this but thought I'd mention it in case anyone else has the same issue, if I find a fix I'll post it here.
I do know I could use the media creation tool to create installation media and upgrade from this, but I'd rather fix whatever's causing this update not to show up.