Hi,
Sounds like cutworms.
If you think it's something in the soil then perhaps the answer is to not grow lettuces in the ground. Have you tried them in a grow-bag raised up on greenhouse shelving? Or even growing two or three at a time in pots on a kitchen windowsill?
The other option is to consider insect proof netting (the stuff used against cabbage root fly) over the plants and growing them through polythene - if it is cutworms then you need to stop the moths getting to the soil and laying eggs.

Posting a picture of the grub might help someone identify it.
If you've never grown chrysanths why do you associate the pest with them?

Thank you urbanite for taking the time to answer. I thought it was cutworms at first but they tend to sever the stems through completely I think. Also from pictures I have seen it looks nothing like cutworm and very much like the lettuce root maggot. This maggot tunnels up the stem causing the plants to die slowly. I have no problem growing the lettuce in a greenhouse or in pots but I lack the space to grow the quantity I need so my veg plot is the only good space I have.
I have read that the lettuce root maggot is the main host for Chrysanthemum and is known as the Chrysanthemum stool miner so I mentioned this in case anyone came back and asked me if I grew Chrysanths.
I have on order some insect proof netting as you suggested and I will give this a try though sealed cloches have not helped. I will also try and get a picture of this pest and post it on this site. I do appreciate your answer and will give your suggestions a try but I think I may have to give up growing lettuce and start buying bags of Florette from the supermarket, shame really but this maggot has really got me beat.
Kind regards
Annie

Looking it up lettuce root maggot and chrys stool miner are completely different beasties. LRM is the larvae from one of the Delia species flies and CSM is a Psila.

I can't imagine how many lettuces you need that you grow them outdoors as well as in a greenhouse.

Ah right, in my book the Vegetable Expert by Dr Hessayon it links the lettuce root maggot and the CSM together but having said that, this book is a few years old.
We do eat hell of a lot of lettuce, a head a day. I only start them off in the greenhouse transferring them outside when they have a good bit of growth. I do this because if I sow directly the pigeons eat them, yes I know, get some netting. Have just put out my last lot, another sixty in the hope I will get some come to maturity as I read, and correct me if I'm wrong, that the lettuce root maggot is more active June/July so I'm hoping the the insect that lays these eggs has better things to do in August. Still waiting for the insect proof netting to arrive though I expect it will be more useful next year now.
I mainly grow salad crops and as well as the lettuce problems I have just noticed that my greenhouse cucumbers have some sort of pest that has stopped the production of the fruits and my tomato plants are thin and spindly this year with hardly any fruit. As a novice veg grower I think I need to get myself an up to date book and do some reading. I thank you for your input and have found it useful.
Kind regards
Annie x

Have you tried diatomaceous earth? I start my lettuces like you do, but when I plant them out I sprinkle diatomaceous earth all around the stem and roots. It seems to work, though I still haven't figured out what insect lays those eggs.