How awful for you James
Definately sounds like Carrot Root Fly. I hope you manage to find some salvageable roots. For future referance, Snadger is right, a covering of Enviromesh is ideal to keep them out, or fleece, well secured all along all of the edges. Also suggested is companion planting of marigolds or onions or garlic to try to mask the smell of the carrots, and/or growing in a raised bed as carrot fly supposedly only fly below 60cm. There's still just about time to sow some more, if you grow a fast-maturing variety like 'Early Nantes', but sow them in a different area to avoid whatever bugs might be still in the soil, and keep them covered up!