Lettuce help please

Growing some lovely lettuce in my plastic greenhouse and also some in the ground but the ones I have in pots outside now seem to being eaten by something small. Just small holes in the leaves, any suggestions for preventing this? Is it the fly? Bit cold yet for flies I would have thought?? Any ideas to keep whatever off? I have wire over them and today due to the weak sun put a piece of plastic carpet protector runner over them to try to warm them up abit.....
When is summer coming?

Growing some lovely lettuce in my plastic greenhouse and also some in the ground but the ones I have in pots outside now seem to being eaten by something small. Just small holes in the leaves, any suggestions for preventing this? Is it the fly? Bit cold yet for flies I would have thought?? Any ideas to keep whatever off? I have wire over them and today due to the weak sun put a piece of plastic carpet protector runner over them to try to warm them up abit.....
When is summer coming?

i'm not sure whick fly you speak of,but as a rule loads of insects will have a nibble.
so if the problem is'nt severe dont worry about it.

No slugs as I have pellets down and as for whick fly( toquote sort of anyway) I haven't a clue as I am too busy working to have time to look. Will rake up the soil idea anyway and see if that helps.
The mixed lettuce leaves dont seem to be affected by the darn things anyway so we should get some calories from them.
Cheers