Hi Karen and thanks for your comments.
What a shame about that bitterness. One other person had this problem and they are on sandy, light soil and it was mostly in warm weather, so lettuce were feeling less comfortable, leaves were drier and maybe then the bitterness is dominant.
As lettuce ages, its leaves do grow a little more bitter but interestingly I fond that leaves off 7 month old Grenoble Red, sown September and planted October undercover, taste superb. So it may be more a soil and summer thing. Try Maryvilla di Verona in summer (Seeds of Italy mainly).
