Should I be feeding my brassica seedlings?

Seedlings don't have that many leaves to process nutrients so re-potting will have given them enough to be going on with for now. If the shrivelling leaves are at the bottom of the stem, these are the seed leaves and will do that, so don't worry.

In pots on a really narrow south-facing windowsill that gets the sun from about 1pm (it's the only viable window I have; everything's crowded round it). The window is single glazing and usually has a bit of condensation on it in the morning. I think a couple of the leaves were touching the window, and wonder if it was cold damage to the PSB leaves. Most of the withering leaves are first leaves, but one is a second.

It might be a little warm for them now ... maybe its time to move them outside to start hardening off.  Mine were sown outside and are coming through now in pots.