Hi Dan, maybe it is less severe than at first sight, as you say the weather has been perfect for the mildew.
I am unclear if you mean it is on seedlings in trays or already planted?
Whichever, I have rarely known mildew to kill lettuce esp. Grenoble Red. I would take off the worst affected leaves and water as little as possible, you may have over-watered. This is now a time when watering can easily be ooverdone and I have watered my plant trays just once in the last five days, because of the 100% humidity. While winter purslane and the brassica salad seedlings are indeed very healthy!
Main point – water seedlings/plants less than once a day from now, eg every three of four days, check first, lift trays and if quite heavy, don’t water! Water in mornings only so leaves can dry before dusk.