Leaf miner in Leeks

I have had to pull up all my leeks today ,riddled with Allium leaf miner. They all looked like they had a bad case of varicose veins with a Leaf miner grup at the end of each purple vein .
  I lost 3/4 of my onions with the same problem,  So i will have to keep both of them coveredfrom  with envirmesh next year , from seed to planting out,

They do seem to be spreading over the country very quickly don't they? 

That is a very good fact sheet Trillium.

One habit in the UK is to keep harvesting leeks right through the Winter and into late Spring when they start to bolt to seed, often as late as May. Some early leeks are planted out by then.

Perhaps we should
  -   clear the crop by March and make sure that all crop debris that might harbour pupae is destroyed before the new generation of moths appears.
  -  delay planting out leeks until at least June.
  -  get everyone on the allotment site to do the same.

The fact sheet mentions some German research which says take off all outside leaves in the Autumn. Presumably that will remove and caterpillars and pupae on those leaves.

....but all members could not agree.

Now that sounds like gardeners for you!