In this case, the period is the same as it was during the fixed term, monthly from 15th to 14th. The tenant notice period is a MINIMUM of 1 month ending on the first or last day of a tenancy period, so if you gave notice on 30 Sept, your notice should expire on 15 November. If it didn’t specify this, its invalid. Some landlords will accept an invalid notice in this way so I suggest you negotiate.