fuck that. cut the stingy runts off and then they'll pay.
you also have the right to claim 12.8% interest for every day (after the 30) that it's overdue.
and £40 in debt litigation costs should you need to pass it on to a debt recovery agency.
sorry, but some clients are willing to chop and change briefs daily, hurrying you along and being really impatient - yet when it comes to coughing up for what you've worked hard on they're suddenly really relaxed. If your a sole trader or small agency, they're likely to take advantage of you all the more as they think you won't take any legal action as it could cost you.
take a strong stance and don't get walked all over for fear of pissing them off.