Join Date: Jan 2006
Car: MG TF 135, MG ZS+ 120
Hot water and a dishwasher tablet. Leave to soak, then empty and repeat. A magic eraser sponge stuffed inside while violently shaking helps. Try using a bent skewer to poke the sponge around inside. If your tank is from a 45/ZS then it won't be possible to clean all of the tank due to its compartments but a combination of the above steps should get close.