ps After discussing your lean issues with two of my skinny family members, we realised if you are only 7 stone, then lifting heavy weights will probably be a non-issue! As in you won't be doing it. Not without breaking your bones. (not literally.) (I don't think!) A grown up who is 44kg won't be going out to chop wood and push the lawn mower? (correct me if I am wrong.)
If bowls of peanuts and almonds, lashes of cream, stacks of cheese, loads of yoghurt and plenty of good quality meats and fish etc don't put the beef on your body, then that is your natural body type? Or, you are ill (compounded by diabetes) and need to see a doctor?
Otherwise, many swear by smoothies and milkshakes for easy weight gain. As a diabetic you would be wanting to keep the sweet down of course, so no bananas, just stevia sweetened creamy milk, and greek yoghurt with berries to flavour? The skinny young man over my left shoulder says "and eat many times a day, instead of just large portions a few times a day." He has also noted that he is not skinny but "lean and muscular", lol. I sit corrected.