Hello,

I've decided to build a bike up from scratch but i'm stuck at the first hurdle.

How do I remove this fork? In this video the man just pulls the second bit. I think I need to unscrew the first big black octagonal nut. However, when I start unscrewing it I can feel it loosen, and then suddenly tighten again. I think i'm running out of thread to unscrew it onto.

Does anyone have any ideas?

I dunno, but it looks like a standard threaded fork with some kind of a threadless adapter stuck into it. Look down the top of the shiny aluminum part and see if there is some tightening nut. You have to loosen it, get it out and then unscrew the lock nuts and bearing preset nuts to release the fork.

rpenmanparker is correct

the top smooth cylinder is a threadless stem adapter
it is held in by an expander or wedge held in by a bolt that runs up to the top

there is probably a bolt that can be accessed through the top
you probably need a 6mm allen key to loosen it
and possibly some way to give it a blow from a hammer to free the wedge underneath
then the adapter will slide out
and the threaded headset components can be removed
and the fork will come out

Ah, I didn't know they made those.

There was a lock nut on the top, but I had undone that before starting this thread. The fork still stayed in place, so it was a wedge stuck in the fork. I got my hammer out to whack it.

I thought i'd be clever and put it on the grass. I dripped some oil into the fork tube, put a chisel in and started whacking it. When I turned the frame over to see if i'd made any progress, i'd knocked soil into the tube! I had to pick all that out with the chisel - rookie mistake.

I managed to knock it out after a few tries. From there it was a simple task of unthreading all the bits.

Long story short: I did it!

that is chisel abuse

If you hadn't taken the bolt out, but only loosened it, you could have whacked the top of it to dislodge the wedge.

Neanderthals did not all die off ..