Sorted!
In case it helps anybody else (as stupid as me) the part literally lifts straight out! In order to do this, I removed the ball to give me a bit more space, but I don't even think you need to do that. Imagine the cleaner is in its upright position, with you hand underneath it, lift the retaining arm of the swivel bar from its clip (the little spring will come loose), keep pulling that arm up towards the ceiling until it's completely free from the yoke. Then wiggle the front bit of the swivel arm until it's free from the brush bar attachment hole at the very front - probably makes no sense when reading this now, but it might just help somebody if they're ever facing the same the same issue.
Regards,
John