Hi sorry its taken me so long to reply but I've been busy the last couple of weeks, I do know a couple of guards that have diabetes but none are on insulin, I belive that you cannot take a safety critical job if you are taking insulin. SWT is very strict when it comes to drugs and the drivers are given a list of things that they're allowed to take (guards for some reason aren't) but the best thing to do is contact SWT occupational heath and they will be able to help you and explain everything to you, but In my honest oppinion even if your current boss does write a letter to swt about your diabetes not affecting your work swt may still not employ you if you are on medication depending on what the rules state, as its all governed by the group rail standards and not just swt choice. I don't mean to put a downer on the job for you but I do hope this has helped you out.
Alan
Alan