"A Heavy landing?" Just how heavy would you like? Burst tyes causing a fire spreading to the tanks which you have not emptied. Broken undercarriage because overloaded. Same result. Too heavy to land at Cardiff runway for runway length. I am not a flyer, but any of the above seem basic reasons for dumping fuel. Swissair did not crash because they were dumping fuel, they did because of another fault, and as you know there are laid down procedures for any likely scenario. Here is a link
http://www.cbc.ca/stories/2003/03/27/swissair111030327