I agree with the majority view on here: let your employer know. It shouldn't be a problem for them and it may well be that they will want to sit down with you and work out if there are any adjustments that need to be made to your working routine to accommodate your condition (such as providing facilities for testing and suchlike, or flexibility in working routines if you have Hypos). Most employers want to help, in my view, and won't be fazed by it. I worked in HR for many years and employed quite a few people who were T1 or insulin-dependent T2 so I guess your employer will have had similar experiences, and will know that, normally, there are few, if any, problems.
Just one note of caution: from a discrimination point of view, Diabetes is usually regarded as a disability, particularly if you are insulin dependent. This gives you protection against discrimination but bear in mind that it is not a "get out of jail free" card. Discrimination only exists when you are treated differently because of a disability (or because of race, sex, sexual orientation, etc.). So, for example, if your attendance at work is erratic because you are off sick frequently, and your employer takes action against you, they will only be discriminating if they act unreasonably and/or treat you differently to anybody else with an erratic attendance record. Not everyone realises this.