As others have said there is no law stating payment must be made after 28 days, if you do not specify payment terms then UK late payment legislation assumes your payment period to be 30 days.
You can either issue a small claim for the outstanding invoice via your local county court of moneyclaim online.
Alternatively you can use a third party recovery agency (like us) to attempt amicable pre-legal collection of the amount. We charge a percentage of the funds recovered dependent on the age overdue. But crucially we do not ask you to pay anything in advance!
Check out
http://www.unpaid-invoice.com for a little more info :)