Being a clumsy oaf, I managed to loose the ball out of the centre of the rear washer jet while I was adjusting it.

I've got a whole new washer jet (the black plastic bit with the adjustable ball in the centre) but I am not sure how to remove the old one and fit the new one.

It looks like the old one may simply pull out and the new one push in.

Is this how they are replaced or does the whole wiper motor/washer assembly have to come out.

Yes, just pull gently out with pliers. Its the centre bit which comes out containing the jet ball.