You really do have top declare it or risk invalidating your cover. Provided you are not taking insulin, other "heavy" drugs or have complications, your diabetes shouldn't increase your premium. I had no increase whatsoever in my premium, and as I travel for multiples of months at a time, my insurance has to be extremely comprehensive.
When you come to renew, just go through the usual comparison sites, but make sure you declare it, or it's likely to be a complete waste of money. The point of claim, when you need help, really isn't the time to learn the gamble you took not to declare hasn't paid off.