Hi Ya, I don't think it is carrot fly as the fly lays eggs and the maggots hatch then leaves red trails where they have eaten the carrot, to get them to disappear could be slugs. Unusual problem and I am very interested in what others think. If it was a mushroom farm how long ago? could be residual chemicals in the ground, Any sign of rabbits? Debris netting would help even to keep larger pests away. The fly doesn't usually show till summer time when the carrots are well growing as they smell the foliage from a mile away and home in on it. Does everything else grow OK? I imagine your cabbages would love it. I know they use a lot of lime in the mushroom compost and that could upset the balance. I use spent mushroom compost but only around my cabbages. what is your soil like? are you on compost from the farm, perhaps a good digging will help. Sorry I can't be of more help.