Hi Oliver
It's under warranty and you didn't fit it.
The split can only be where the 2 halves of the tank are 'welded' together.
It will only get worse - get them to replace the whole tank.
If you can't spare the time and long trek to the supplier, suggest you contact a local plastic bumper repairer as they do plastic welding. It's a special gun they use. Wonder how it would work with a damp tank?
Speak to your customer Carl at Body Right just round the corner from me- he will point you in the right direction I am sure.
You guys must have been desperate for a baby sitter on Friday -using Hazel.
Bruce