[andyf75]

Hi, I've just set up one of these:
http://www.greenfingers.com/superstore/product.asp?dept_id=200360&pf_id=LS7730D
and have put in a few lettuce and spring onion seedlings that were growing in trays. Now I'm starting to wonder whether they may get too hot under the plastic during the warmer months. Its in a semi shaded position. Any thoughts? would netting be better for the summer rather than plastic?

[viettaclark]

You didn't specify which product you are using but lettuce and spring onions tend to grow well in the Spring and Autumn because although they like some sun they don't like being too hot.
Having them under plastic over the Summer (if we actually get one!) could lead to drying out and bolting so it might be better not to do it.
Netting may not be necessary. Pests are mainly slugs/snails unless you've got squirrels that dig everything up like me! Birds and cabbage whites are usually not a problem.

[seaside]

I can think of a lot more plants than lettuce and onion that need protecting from the air through the main growing season.
I would only protect those two crops from the air for a very short while, if at all, while they are establishing themselves. Slugs are the issue with lettuces, maybe a rogue Thumper rabbit. I don't know anyone who protects onions after they're up and running.
Scarce protection resources should be deployed elsewhere.