thank you , dx

Hi All, i have an account with santander with an overdraft of 600 paunds. used the 450 paunds and totally forget for a very long time like a 1 year , then with a charges the amount reached 1101.06 , passed to a collection agency . in april i settled an agreement to pay the full amount in 6 months , but i couldnt sleep at night so i decide to pay the full amount by july. and finished. but when i checked my credit report is still showing as default and on the details showing the only first instalments i paid in april please help any suggestions ? what can i do ? since i never been throught this whole my life