yes the collars don't really stop white fly. they fly and then lay their eggs and before too long infestation.
I suspect the only solution is to spray with a systemic insecticide designed for food crops and that will kill the white fly.

Collars are used for Cabbage Root Fly. White Fly is something different. I would go for a contact spray suitable for food crops. Difficult to get rid of when other allotment holders are growing brassicas. I have noticed that even in severe winters the fly survive even when the plants are frosted.

If you only have one row you could try one of the tunnel cloches that keep insects off, to prevent the fly laying eggs on them.

My neighbour down the allotment is an eco friendly nut who refuses to spray her plants with anything apart from fairy liquid or cows milk or garlic or liquified rat droppings or whatever else she could think of. All that seems to happen is that everything she plants gets destroyed by natures carniivores.

I was getting so fed up of her "bugs" popping over to my plants I accidentally sprayed them with some proper stuff.
I didn't have the heart to tell her when she came over so chuffed to tell me that her witches potions had worked.

The moral being, be as eco friendly as you like so long as what you aren't doing doesn't affect anyone else. Or else.

That's the way to do it Graham. But be careful she doesn't twig it and turn you into a toad!

Organic soap solution has worked for me, but you have to spray thoroughly and frequently to get ahead of them. Also try planting plants with small flowers, such as Sweet Alyssum, or Linaria maroccana nearby, to attract parasitoid wasps.