Thank you again for your replies. I dug a few tatty leeks and indeed there were holes in the upper leaves, and I found a single grub which looked like the ones you find in older peas. I conclude that this is the result of 'Leek moth?
This is the first time in 35 years as an allotmenteer that i have seen this let alone suffered from it. I will try and cut the tops down as gray and Ace suggest, but with the season geeting on, I will Soup a lot, cut down and see how it goes.
Thank you all again!!