Andy your biggest problem is that their droppings look a little like peas, so the pigeons might eat them, apart from, as Robert said, the disturbance they cause at night whilst under the loft. I do not know what they carry that is transferable to pigeons, but the normal suspects would be tapeworm, roundworm, coccidiosis, etc. For me, prevention is better than cure, so get that dug out there at night. :D