Bad luck fella.  I was just thinking about you the other day, assuming from the 'radio silence' that all was well.

I had to replace the rad on ours before I realised the HG was causing most of the problems.  I took it apart a bit out of curiosity, and took a couple of poorish pictures.  I didn't look at it very closely at the time with respect to the failure you've seen, 'cos ours cracked in the plastic just near where the top hose attaches, but as far as I can see/remember, the  main metal section has a load of fingers that crimp over onto the plastic parts at each end.  Looking at my pics, I can only guess that the white rectangle around the perimeter of the alloy section is some sort of compliant material that the outer rim of the plastic sections seal against. 

Edge view of alloy 'main' part:


Inside view of plastic endcap:


You can see in my pic of the plastic (other end from your prob) that it's distorted a bit laterally, a bit like a plastic camcover ( ).  I guess a bit of that and a bit of distortion in the plane perpendicular to that, and the metal fingers are going to move and loosen, leading to what you've got.

Never heard of it happening before though.

I doubt you'r egoing to be able to fix it in any lasting manner.