This morning's drama is now resolved.
A white van with a trailer-full of tanks pulled up about half twelve.
Pumped the remaining oil into the new tank, disconnected the old tank, pumped the oil back into the old tank, lifted the new tank into place, connected it to the pipe and pumped the oil into new tank (again). Sounds like a modern version of the getting the chicken, fox and corn across the river puzzle!
Looks like when the flange was trimmed in the factory, the knife went into the plastic, weakening it, and eventually creating a crack.