I have two really old clumps of Mamm, and one is definitely starting to rot from the inside of the clump, and the other may be getting there. Just wondering if this is normal in old plants? I was talking to a seller from the Cactus Society at the show last year and he was telling me that he just broke up a noto clump that had 'deied underneath' as he put it.
Is it good to break offshoots off clumps as they get bigger?
Also any cutting tips? I'm a bit nervous about breaking them up in one go but I think at least the first one needs to be done

Why not see it as exploratory and learning experience. You don't know the problem exactly know, but you will know when you cut it through.

I think it mostly comes from rotting in the plant base. Maybe use more gravel in the base, and a more free draining material as substrate, also protect from rain more....

Nah the soil I use is top notch, I have been reading up on mamms and there was some notes on rain getting trapped under the spines. I thought it was the size, it was about 30cm across, and a bit pot bound when I got it back.
I divided it this afternoon, it was rotted at the base so I have about fifteen plants drying out now, I just cut them above thr rot and the other tissue looks good.