thanks all, i didn't actually use gutter silicone (or no-more-gaps) just bored and thought i'd post it up and see if I'd get a bite
I went with Stu's 515 gasket idea, cause I already had some, and the spigot on the tank had been shortened slightly due to previous damage. I also put a nice new rubber seal in between the tap and tank and everything seems swish.
If, however, you notice a fireball/plume of smoke from the pre78 track at CD8, that'd be me. (thanks heaps Stu
)