my potted petunias have gone very leggy this year early help what can i do

If you cut them back now, you will get another lovely display later on.

cut them back to how low please

I've never cut them back, just stopped growing them because they do what you're talking about, Millie. I only grow the Surfinia trailing petunias now, and they're fine cos they're meant to trail.
If you are going to cut them back (and I suggest you do, because I know how ghastly they probably look, it's worth a try) take them back by about half, so assuming they're 8inches tall, down to 4. I don't know what'll happen, but they should reshoot eventually and form new flowers on the new shoots.

yes cut them back by about half, and give them a feed of tomato food, that should give a good show in a few weeks