Please supplement your question - it has become truncated by the word limit! Hit "Supplementary question" to finish it off!
I'd say that yes, you have to pay it. You agreed to buy a product for a set price, and the store offered you an interest free scheme to facilitate this, so unless you had received a letter telling you that your payments were complete, you don't really have much evidence that they said that the account was now closed.
I'd be tempted to suggest to them that since they had made a mistake - and that they will be able to see that you actually paid one payment before it was due (or at least an extra £15 that you wouldn't normally pay) that they let you restart payments now, without any late payment charges or penalties.
But I do think that you should be obliged to make the remaining payments.
Sorry - that's probably not what you wanted to hear!