After your advice I have got a plastic tank as it should stop any problems in the future. I just need to get the fuel pump assembly (+ remove the regulator) and I was wondering if you happen to know if the Monster 2004+ with the plastic tank uses the same as the ST2 as the sizes look the same (an example below):
I know the ST2 size is about 120mm (diameter), I just have not got home to check the size of the plastic tank yet... If it is then I think i am lucky... though i know it might not be as simple as what i think..