Hi Beth,
I always tell the check in staff about the supplies in the case and then tell the security staff about the stuff in hand luggage when I go through the metal detector. I've always found if you tell them up front they don't have a problem. Seeing needles go through the x-ray when you've not mentioned it could cause a raised eyebrow in my opinon which is why I always let them know first.
I carried a letter for years and never once got asked for it. The moment I stopped bothering I got a whole load of hassle at Eindhoven airport. It was obviously a slow night (don't think they have any other kind at that particular airport) and the security folks had nothing better to do. For that reason I make sure I have a letter again nowadays.
I never, ever check insulin into the hold, but do have spare test strips, needles, lancets, pens etc. I have heard tales of things freezing in the hold, which could potentially kill off the insulin. More to the point, I have had several occasions where I arrived and my luggage didn't. Last time it happened, my colleague was in the same undies for 2 days

. I very smugly changed into the spares in my hand luggage and injected the necessary. For that reason, make sure you have enough insulin etc for the whole trip in hand luggage. If you want to carry spare in your case as well, fair game. I'm sure someone else here will point out my scaremongering as to the temperature down there. But don't leave ANYTHING in your case that is an essential just in case it gets lost. That's my .02 cents anyway
Gary