Heki 2 leak

Hi folks,
 Couple of questions here please. First one is my heki 2 roof vent has developed a leak, i have taken it back to the dealer and they said they will take the heki out clean the old sealent off and reseal. Can i confidently now this will resolve the problem? Secondly, any suggestions as to what will remove the water mark (stain) that the leak has left in the roof fabric!

Some ideas here....
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