I'm very close to just giving up for now tbh, it's just silly now.
Oh, and I've got a bit of a head from all the "I'll just try this"...
I think we're in an acceptable position, but I'm going to investigate further later, starting with measuring bloody everything inside next time the tanks come apart to check for variations that may affect juice flow or something - and I think I'll be looking more at the electrical side when I get around to it.
Y'know, I was really hoping for "oh yeah, it's this" and sorting it easily, but I just can't have easy problems