Good info here:
http://money.guardian.co.uk/howtocomplain/stor y/0,,1733350,00.html
As ever, first impressions do matter, and it will help you to feel more confident if you feel you look the part.
No need for a suit, but a generally clean and smart appearance. If you are comfortable in a suit, fine, wear it.
As soon as you can, go through all your paper evidence.
Put it in date order, and staple loose bits of the same correspondence together.
Then number each piece, in pencil. This will help you to keep it all in order and stop you getting flummoxed.