The old cable will be frayed and jamming, probably where it leaves the metal conduit and enters the bowden tube. If there is an angle here instead of a smooth transition, that will be the cause.
If your lever was operating the clutch rather than just taking up the slack you wouldn't be moving it as easily as you are in the video. You didn't forget to install the clutch release bearing did you?
Yes it does. There is no spring on the pedal, it's all sprung by the shaft spring on the Gbox. In that case, to me it looks like you haven't adjusted the cable anywhere near tight enough and that it's too long to adjust without stacks of washers taking up the slack.
Is the arm on the gearbox moving within a slack zone then? It doesn't look like it is moving far enough to engage anything. Should I be able to engage the clutch by hand or not very easily? Never done a cable or clutch before
Compare the cable length with the old undamaged one? However as you have retrofitted an earlier Gbox, there may be no way round it apart from the stack of washers between the top of the bowden tube and the bracket on the Gbox, or if that makes the S bend too much, washers under the wing nut.
Not a hope by hand, those clutch finger springs stop you clutch slipping! You're just taking up a bit of the weight of the pedal so far and the movement on the box arm is indeed slack, not clutch action.
You can also try, slackening off the cable then undo the 2 nuts on the cable bracket and knock it towards the back of the van, this removes some cable slack, it does sound like your cable is yet to operate the clutch.
It wasn't even engaging @zed @davidoft @Dicky I just managed to wind it up and now it's working. I didn't realise the movement I was seeing was just the backlash. Cheers all. Another problem halved and solved. Be an expert soon.