Re Alliums, of course you can. All you need to do is leave the spent Allium flower on the stalk for another month or so and the small black seeds will fall out when you shake them. They take 3-4 years to produce flowers.

Remove all Brassicas from that part of your garden.
Burn the roots, or put them in your wheelie bin to go to landfill to be burned. Dont leave them lying about.
Then dig plenty of Lime into the soil. Also in the rest
of your garden, as the spores transfer on your boots.
Dont grow Brassicas on that area for several years.
Years ago they used to say 20 years, but it was only
3 years when I used this method.
The spores also blow about in the wind, as my
infestation came from the other end of the allotment
field, and I hadnt walked on that plot.

many thanks kildermorie and dianebulley,dont know if i can wait 4 years for more alliums.think i'll buy the bulbs,and will get cracking on my plot tomorrow