Just to update this, The vinegar soak worked ok but still needed scrubbing off, I then got some acid that they use to clean the inside of concrete mixers with, real nasty stuff smokes when you open the bottle and the fumes would choke you wild handy, about 200ml put in then half filled the tank with water mixed in, sealed up and shook about well, next day opened it up and where the mix was the tank was like brand new inside, so I then fired a litre in and filled it to the neck while swishing it about, 2 days later tank is like brand new inside Gave it a good rinse out with the hose and then petrol straight after then fitted it