The end of the loom (yolk end) slots into a groove and the prons for the cleaner head also sit there, they correspond to the head, that must say seated, a slight movement makes it come off unfortunately.
There is a clear cog thing on the left inner part of the yolk, make sure it's tilted upwards as the yolk is being fitted on and it should fit in on both ends securely, then screw the screws in, 3 on the filter side and 2 on the left side when the semi circlular white part is on the top of the side end of the yolk. Then put the washers on and the ball side hemisphere goes on, then on the filter side the filter holder goes on with about 4 screws IIRC, filter in and filter cover on, job done! :)