Gary,
Was just catching up on your posts over at the Wooden Boat Forum (I'm not a member, so can't post there), a header tank is the way to go, but no need for it to be stainless steel. A polyethylene tank that you can buy of the shelf (I suspect Whitworths would have them) would work and be much cheaper than SS. Alternately, if you can make the time, a plywood, epoxy & glass make great diesel tanks with the added bonus that because would is a good insulator, you get minimal condensation inside the tank.
On LC, despite being a steel boat I have plywood epoxy main and header tanks and have had them for more than 20 years. They are light, stiff and cause minimal condensation. I can highly recommend them.
PS. Went and had a look at Whitworths, did not see any PE tanks. If theres a West Marine in Oz, I know they have a whole range of them.