Cleaning brown stuff off bottom of iron?

[GwendolineMaryLacey]

I know there must be threads but asking again is quicker :)

What household substance could I use to clean it? I need to iron DD1's school dress and it's already left marks on one of them.

I was recently recommended to use a cloth soaked in vinegar or lemon, or a product called hot iron cleaner

Thanks I'll try those. A paste made from baking soda and water was my first failure!