Onion & Leek Moth
It seems that the Leek & Onion Moth is a big problem over the country these days, but luckily we have not had the problem yet in Stephanie’s Kitchen Garden in our corner of Norfolk (touch wood!)
The fact that your leeks/onions were grown in the same place two years running may have contributed to the problem so crop rotation is advisable, you mentioned in your Email that you plan to do this so this will help immensely.
Unfortunately adult moths overwinter on plant debris so good housekeeping is essential – destroy severely infested plants and material and burn piles of leaves and plant debris before winter to drastically reduce the number of overwintering adults. Dig over the soil once harvested do disturb any overwintering adults and pupae. Carefully check the leeks in spring for caterpillars and destroy if found, and you may find that applying a organic contact spray the same time will check the progress of any caterpillars. The female moths can be prevented from laying eggs by covering susceptible plants with horticultural fleece
Harrod have two products on the website that you could use;
Insect Killer Spray relies on Pyrethrum as the active ingredient so should do the job on leek moth caterpillars too.
Savona which is an organic fatty acid concentrate used as an essential pest control liquid.
I can understand that you don’t want to go to the expense of protecting the whole area but it would certainly help. Take a look at the Insect Mesh Protection netting and the Ultra-Fine Insect Mesh Netting - you will find these cheaper than the recommended Enviromesh.
Good luck with your leek crop next year – I would be interested to hear how you get on.