The DVLA guidelines on assessing fitness to drive. This is written for healthcare professionals, but clearly lays out the criteria for driving with diabetes. The pdf is for all health conditions; you will need chapter 3 for diabetes!
Don't be put off that this is a guide for professionals as it is very helpful to patients as well. An updated document to show which standards are required for drivers with type 1 diabetes, this is especially useful to those newly diagnosed or anyone applying for their first drivers licence.