store card cancelled continue to pay of balance

i have a store card which i have had for 20 odd years. last yr my husband became unemployed so money was tight. my balance on my store card was 900 and i had experienced problems making payments. the company stopped the card and it is now null and void but i continue to pay off the balance which is now 1600 - £700 of this is penalty charges and interest. A friend informed me that if the card is cancelled i should not be paying interest and just the balance instead. Can anyone throw in light on this thank u