As to the carrot fly and wishing to stay organic and not use any pesticide the only solution is to grow onions and garlic on either side of the crop. As to stunted carrots I can only assume that the soil has not been loose enough for their growth. And as luck would have it, left in the shed by the previous owners was a very rusty, but still usable, round metal sieve.
I then built a little raised bed, about the size of a standard skirting board height, out of some pallets left lying around.
I sifted the soil, taking out all stones and lumps, over the bed until it was full. I think I put four rows of 'Sutton seeds Autumn king 2' carrot seeds in. This variety seems to be the most available in the shops so I hoping I cant go wrong with them. If this works out I did see some very colourful purple carrots that I may try next year.