Thanks everyone for the advice to date. I called Selley's tech line and was told that none of their products stick well to polyethylene but All Clear is the product that will have the best hold, and to allow 2 weeks for drying.
Try west system g-flex epoxy. A toughened epoxy designed to work with tough to glue materials like polyethylene. Good shear strength with just a rough up with coarse wet and dry - even better if you "flame" the poly before glueing (don't be scared - its easy enough if your careful).
I'd mix it with some collodial silica to thicken it up and "pipe" it in around the joint with a heavy duty plastic bag with the corner cut off. Its a bit more expensive and mucking around than with all-clear however.
The foam bulkheads are usually cut slightly oversize so there is outward force on the joint. The silicone won't form a chemical bond with the PE but it should still work well enough, or at least as well as foam bulkheads ever do.
Interesting tip on the epoxy Shawn, I might experiment with that myself. It would have to be cheaper than 3M Scotchweld, which works well but would be a fairly expensive fix for sealing a bulkhead.