Yep, the lines running down the length of the car are fine, the rust starts at the connection to the hoses on both sides and works back the pipes. I'm gonna replace from round where the driveshaft meets the diff so at least I know the bit I can't see above the tank is good as well. Tank straps are rusty at the bolts as well so no harm opening them and giving them a bit of tlc. Cheers fellas, might actually take photos of the procedure for anyone else with the same problem...