In Brisbane Australia we dont have that problem it is the other end of the scale at 32C and trying to cool down so the plants dont bolt and go to seed.

Well, the results are in. Lost about half my lettuce, have a little leaf curl on the survivors. None of my basil or cucumbers survived, had to start them over. Parsley and Rosemary survived pretty much fine, and oregano was 50/50. SO, the upshot is, if you're doing NFT, you want, at all costs, to AVOID A FREEZE!!! LOL
The addition of some heaters to the greenhouse and heating the water res properly have handled the far worse weather we've had recently. The big test will be the next two days, when we're supposed to be between 6 and 10 degrees below freezing here. Fingers crossed, and kerosene on hand, just in case...