Another source of problems is the joint between the tank and the gunwale on a MK2 (I don’t know if this also happens on other marks). Newer boats have an aluminium strip and self threading screws. Despite the screws the joint will still start leak after some years.
Though my solution will not win prizes (and probably earn some serious criticism from Colin) I have fixed this by squeezing some Sikaflex on my fingers and then, with my arm through the hatch, smear it inside the tank on the joint. Since nobody sees it I don’t care how it looks and I make sure there is enough of the stuff on the joint. I even try to push it in between the two layers with my fingers. Though intended as an emergency repair it has held remarkably well.
BTW use white Sikaflex or be prepared to have black fingers for a week or so. Or maybe I should try surgeons gloves next time?