The reason most Haggis split when reheating is because nothing has been put in the bottom of the pan to prevent the Haggis from touching. Even at a simmer the heat on the bottom of the pan is too intense and that will split the casing. A rack of some kind on the bottom of the pan will prevent this. You can also leave the Haggis in the shrink wrap it came in (or wrap it tightly in plastic if it didn't) to help. Also remember 30 to 60 minutes is all that is required to heat all but the biggest Haggis. It will already be fully cooked so you just need to heat it through.

That said, the above method is not what I use. I don't simmer the Haggis I steam them. I remove the shrink wrap and then tightly wrap the Haggis in plastic wrap*. In the bottom of a large pan with a lid I put 1 inch of water. I then place a rack in the bottom of the pan. Bring the water to a boil, turn the water down to a simmer, place your haggis on the rack, put the lid on and then remove the haggis when it is hot, 30 - 60 minutes depending on size.

The best part of this method is if you need to heat up a number of Haggis at the same time the method can be done in the oven. Place a rack in a roasting pan, fill with an inch of boiling water, put your haggis in, cover tightly, place in a preheated 350 F oven and wait. Similar timing is used.

As a caterer I have reheated well over a thousand Haggis using this method and never had a problem with even one.

* I re-wrap in plastic wrap, rather than use the shrink wrap because I find you are more likely to get burned getting the shrink wrap off and so the process is slower. I fact that matters to a caterer.