Cleaning a large copper planter. What to use?

[erebus]

I am keen not to go and buy a £6.99 bottle of gunk just to polish what is, after all, a planter!

What should I use? Tomato ketchup?
tia

A bowl of salt and quartered lemon/s, depending on the size of it. Dip lemon into salt and rub and rub again. This will produce a lot of black "dirt". Once satisfied, rinse and polish with a dry cloth.

P.S. 40 pence?

Thanks. I'll give it a go tomorrow.