Take a square of baking paper big enough to line your tin.
Fold it into quarters.
Now fold the quartered paper in half to make a triangle, then in half again the same way, keeping the edges of the paper to the outside of the narrow triangle - opposite the sharp angle that will be the centre of the circle.
Now hold this point over the middle of your tin and looking at where the edge of the tin is, cut along there in a curve.
Unfold and voila! - a neat circle that should be the right size. If a little big, re-fold and trim a little more off. Neat trick, huh?!