Talk to me about Bricks

What's just been brought up is to consider what you're looking to get out of the workouts. What you've been describing is more race simulation due to the size of the splits. The brick part of it is more in the transition from one discipline to the next. The actual change of shoes and stuff only mildly, but more so the appropriate muscles firing correctly. For this, much shorter splits would be appropriate and then cycle through them several times. That doesn't mean all 3 necessarily. Just go back and forth between two should work. Should only need to do a few hundred swimming, a couple miles on the bike, and maybe a mile or less on the run. Not trying to recreate the race, just going long enough that you're into the split you're doing.

Considering that this is for a HIM though, it shouldn't be as necessary to really push on this. It's a long race so that intermediate time won't take up a big percentage.

As someone said already, the big thing is getting accustomed to switching from using one set of muscles to another, although you'll also want to practice transitions as you get closer to your race. But for the switch between muscle groups, generally it isn't that challenging to go from the swim to the bike - the challenge is more aerobic (especially in shorter races) in that you really get your heart rate elevated when getting out of the swim and running through transition, so you'll have to get used to that feeling. But I like to do that within bike sessions for example by doing some high intensity intervals and then doing long "recoveries" at a brisk speed (race speed) to teach my body to get my HR down even when I'm going relatively fast.

The bike-run brick is the challenging one, so that's the one I do, and I do it weekly. The concept of jumping back and forth is really interesting, and I still hav to try that out myself, but it could be well worth a shot!

I have suffered with severe cramps following T2, as I was well hydrated and I'm reasonably fit I sure this was down to not brick training properly.

From what I've read it seems as though the contracted nature of your quads and calves during the bike phases have issues extending as you begin the run causing cramp. There are other factors but this appears to be the main one.

I've put long bike/run brick sessions into regime but will defo try the run/bike/run/bike approach - thanks!