Ok found the issues.
On the pipe going from the front to the back I did find some little spots of UV due. However one area just had loads due to compelte corrosion. Observe below.
I highly recommend that anyone with an LS430 and rear air con gets their car up, check out the pipe and put extra padding and covering around it to protect it.
To fix this, instead of dropping the tank ,prop etc etc. I am going to first go down the pipe leak self amalgamating tape method. Very good stuff for leaking plumbers pipes etc. Should do the job here at least.