Luckily for me I don’t think that joint has failed. Imagine looking into the gutter from inside the boat (looking parallel to the transom) under the side deck. There is a flange from the tank that forms the back of the u-shaped gutter and it joins to the transom as described in the other mails. My damage is in the front edge of the U-gutter. Difficult to describe.
Anyway it’s a right pain in the wotsit because I only have a small inspection hatch so I don’t think I can reach this from inside.
My hatch is also leaky but I’m confident I can sort that out with some sikaflex. I can get some pressure but you can hear the hissing from the corner and if I stop puffing it fades away after a few seconds.