. has just had someone else in her flat start with symptoms and so the 14 day clock re-starts.
The ten day isolation period for the new person starts from them developing symptoms. According to the Government publication, the isolation period for household members is 14 days from the first person becoming ill and ends after the initial 14 days if they do not become ill themselves:
Stay at home: guidance for households with possible or confirmed coronavirus (COVID-19) infection - GOV.UK
www.gov.uk/government/publications/covid-19-stay-at-home-guidance/stay-at-home-guidance-for-households-with-possible-coronavirus-covid-19-infection
Stay at home
Stay at home for 14 days. This is because you may have been exposed to the virus and could pass it on to others, even if you don’t have symptoms. Do not leave your home unless incertain circumstances, and only exercise within your home, garden or private outdoor space.
Your 14-day isolation period starts from the day when the first person in your household developed symptoms. If you are isolating because someone in your house has had a positive test result but does not have symptoms, your 14-day period starts from the day their test was taken...
... Returning to your normal routine
If you remain well, you can return to your normal routine at the end of the 14-day period. You do not need to isolate for longer than 14 days, even if other household members develop symptoms during this period. However, the person with new symptoms should now self-isolate for 10 days. People in the household who remain well after 14 days are unlikely to be infectious.