Andrew have you tried sourcing good northern raised queens from respectable breeders here in the North East? I have found some success with buying queens from local breeders (that treat for mites) and incorporating them into my apiary which has now been without treatments for 5 years.
Master Beekeeper (EAS) and Master Gardener (U Maine CE) www.beeberrywoods.com
Yes I am in Plainfield. Its a small world, Where about in Cornish did you live?