Hi knocker, sorry I missed this, yes it looks to me like a bit of nute burn too. The Mills Lightmix will contain enough food for the plant at this stage and for a while yet so no need for extra nutes at the mo.
@Deckard I would imagine it was pretty hot in that propagator so it sounds as though your seedlings may have been cooking a little bit. If they have proper leaves and it's nice and warm in the day take the propagator lid off, they'll love all the fresh air and sunshine!