Yes, it's worse than I thought, there's no saving the poor leeks. I have neglected them and this is the comeuppance!
So I tried squishing all the onion fly larvae / maggots and thought I was doing pretty well but decided to dig a couple up to see how far the maggots bore into the stem. Unfortunately it's a long way and at most there's only a couple of inches of stem unaffected. Plus there's no way of getting all the maggots out without pulling up the whole leek.
So my next plan is to pull all the leeks up and just salvage the bits which haven't been munched, squishing all the maggots along the way. This will take a long time though as I did a few tonight and it's slow going, making sure you've got all the nasty maggots. I guess most people burn them.
Hey ho - next year I will probably get some netting, which is apparently the only prevention. It seems as though everyone on my site has got the same problem though - one guy even worse than mine - I don't think he'll get anything.
A friend with a plot on another site in the city has succumbed to leek moth. However a couple of people at work who live out in the countryside a bit haven't had any problems.
Hope you all are more successful than me!