how do I stop my seedlings going leggy

They are not getting enought light, that is why they are leggy.

Try pricking them out once you've got a set of proper leaves, after the initial cotyledons (first rounded leaves) have appeared, and then placing them in a lighter, but not sunny, spot. In a coldframe is ideal

Not sure my answer's that clear - I mean to say you need two sets of leaves on the seedlings - the initial rounded ones, and the "proper", 2nd set of leaves.