Without further details of the insurance company's argument it�s not possible for us to comment on whether you owe any money or not. But at least you have 7 days to 'look into it further'. Tell the insurers you want a written explanation of how they arrive at the figure and then you will pay within 7 days if the amount is agreed; but if it turns out they were wrong and yet but that if they still call in the bailiffs you will take the matter to the press and insurance ombudsman.
But I have a feeling the insurance company must be sure of its ground. If you cancel early you don't normally get a pro rata refund - so if for example you had paid 6 months premiums and then cancelled after 6 months you may find you have to pay some more.