onion eel worms

Last year on our allotment the leeks had tiny pink insects amongst the leaves, making them slimy.

This year, our onions,  which have been grown from sets bought from an allotment shop, supplied by a reputable mechant,  seem to have a problem of flatened screwed-up leaves.

We always rotate the crops each year.

The onion bulb does not swell and not all are affected, but someone suggested it may be caused by 

Onion Eel Worm.  

How is this treated? Do the affected onions have to be destroyed. 

Can we plant any of the allium family on the same ground in the same place in4 years or do we have to wait longer?

Does the worm live in the soil or come I  with the onion sets?