An individual has to tell HMRC that you are no longer a partner in a business. HMRC are only as good as the information they are given.
A partnership has to complete a tax return for each tax year disclosing the accounts figures and profit shares for the accounting period ending in that tax year. There are also boxes for a partner's start date and leaving date which are filled in where either or both those things occurred in the tax year concerned. Somebody must be completing and submitting partnership tax returns. If they weren't you would be receiving penalty notices.
As a partner in a business, you also have to complete a personal tax return to disclose your income from the partnership and any other income you have. If you completed and submitted your personal tax return for 2011/12 and did not put a date of leaving in it, then you need to tell HMRC
or tell the accountant of the business if there is one to do it for you. Otherwise, you will continue to receive tax demands for tax and NIC which may not be due. You may also have 2012/13 and 2013/14 tax returns to complete.
You should find out from the remaining partner or the accountant what is being disclosed as your profit share in the partnership tax returns, if anything. Technically, if you no longer consider yourself to be a partner, then the partnership no longer exists and your former partner should be disclosing the business income as a sole trader. However, if she is doing that, you shouldn't be receiving tax demands.
Once you have discovered what is going on as far as the reporting of figures is concened, it should be easy to get things straightened out with the tax office.
I hope this helps but let me know if you have any further questions.