I had the same problem on my '03 V8 when purchased. There was a small crack next to the screw, probably caused by over zealous tightening.
I used a soldering iron for heat and fused the material either side of the crack together. Although probably a different plastic than that used on the tanks, I used a plastic saddle clamp from a watering system as the source for a small quantity of black PVC 'filler'.
Has worked like a charm for 18 months so far.
I've successfully used this technique a couple of times before on cooling system expansion tanks that develop small leaks through cracks and splits.
The trick seems to be related to the amount of heat applied. Too little and the bonding is incomplete, too much and the original material becomes very brittle as it oxidises and bubbles.
Well worth a try as it's a shame to throw out an otherwise OK part for the sake of a mm size defect.
Andrew
