I have type 2 diabetes and have recently retired from being an ATCO.
My understanding was that so long as it was controlled by tablet/diet/exercise and subject to additional testing by the CAA ( effectively a second medical carried out by the doctors at Gatwick ) I was good to carry on.
If it became neccessary for me to have the diabetes controlled by insulin then my medical would have been revoked.
For a full discussion on the subject I would suggest a chat with your local AME.