[Janeymiddlewife]

Leek moth   
Rotated my leeks this year to avoid rust (which they did), only to have copped it with leek moth, consensus seems to be that there's little point in spraying, keep feeding & watering and they might survive - why do my neighbour's look OK and mine are really sad?

[GreenOwl]

Ah!  That's what it is.  They were looking lovely, then a bit tatty, then we realised they were bug-ridden.  

Thanks Janey.  Just going to goggle it.

Edited to say:
Didn't google, searched this site instead   
Advice seems to be to cut off affected foliage, even down to ground level and they may yet grow some more (advice given in previous Septembers) OR dig up and salvage what you can to freeze.

[Ropster]

I had this last year, just cut off the worst bits and let them get on with it, still got some edible leeks