Well I managed to get it coughing this morning, simply because I only put a capful of fuel directly into the carbies and hit the starter. Looks like she'll be OK when I get the proper fuel tank on.
However, the fuel tank is really, really rusty inside. I can put my finger against the raised middle bit on the inside and get orangy red powder on my fingertip. Sounds almost like there is fine sand on the bottom too.
I did a bit of reading on tank sealers and the like and it seems that there is a product called 'RustEco' in the US which is apparently very good.
It'll cost me around $130 for a gallon delivered, but I figure - do it once, do it right.
It was very reassuring to hear the engine turn though, its made my day!