I contacted KingsSeed and they have said not before the 3rd week in May at the very earliest! Celeriac all planted out but will hold fire with celery which is not quite as advanced.
Some varieties are perhaps just more sensitive than others.
I regularly try to grow Sunflower Hallo. Had success with them the first year. Now, I'm lucky if 50% germinate - but the Mixed Cut Flower seeds all came through in the same conditions