hactually it is that simple.
Call SLC you will get through to a grumpy old scottish lady (I think shes the only person working there), ask to make a one off payment and put a stop on the rest of the year. You will have to send copies of payslips from April to now to prove what has been paid thus far against the balance you had in March/April and then you can clear the rest and have the stop put on the wages.