Jason.ruff wins the prize!!!!!!!!

It was a collapsed cell in the battery!

I never would have thought a car wouldn't start when attached to a good battery via jumper leads, regardless of how bad the battery was, but I chucked a new battery in and the van fired straight up. It's charging at idle and all appears to be good.

I don't normally get out of it this easy, Murphy's law usually rules and my problem is the worst possible one, but this time I had a good one.

Thanks to everyone for your time and suggestions, I tackled the job with a lot of confidence that no matter what it was, I was going to win.

In fact, we all win, because most of us have just gained some more information that may be of use down the line.