Chain "jumping" every few crank rotations After a few pedal-strokes the rear derailleur completely gives out and the pedals drop to catch up.
After much googling it says the most common thing is rear derailleur adjustment is needed, so I have done that which was pretty straight forward but the problem persists.
Video: https://photos.app.goo.gl/XnrwqW4WatoFc7ee8
Excuse how shaky it is, the bike is hanging on a washing line...
I followed this guide when adjusting:
After hours of thinking it was indexing, I slowed down and watched the chain only to learn that it was a single stiff link in the chain.
Thanks to @Warren Burton who helped me get there.