are they white fly? if so then spray with an appropriate insecticide. or fungal?

Thanks Seaburn, to me they do look like whitefly but unlike the ones I used to get in my g'house years ago these ones don't fly off if I disturb them, I' ve never lost all the leaves like this.....

Do you have Rape fields growing nearby ? Rape Beetle is an awful nuisance on Brassicas. They have to be washed
6 times in the winter to get rid of Rape Beetle fly.

Are the white 'flies' actually the shed skins of the greenfly/aphids that are elsewhere on the plant? Whatever, it's a serious infestation - impressive. I think, despite my reservations re pesticides, that a spray of something noxious is required, then a clip back and tidy up, a good wash off and watering and wait for nature to help out in a positive way! Dispose of the rest of it.

Thanks all, I have sprayed the bush, checked everything growing near to it and no signs of it spreading to anything else, got to hoover up the leaves and wait and hope.....