Question about "self isolating" diagram in Boris's letter

[BruceAndNosh]

Apologies if this has already been asked but we only got our Boris letter today.
This diagram explaining household isolation...
A catches CV from outside the home, the rest of the house start isolating too in case they catch it from A.
B catches it, assuming from A.
C seems ok but remains isolating just in case

Then after well over a week, D gets symptoms and prolongs their isolation.

But shouldn't C start a new period of isolation when D gets sick in case C catches it from D?

This bit explains that part of the diagram:

Should a household member develop coronavirus symptoms late in the 14-day household-isolation period (for example, on day 13 or day 14)the isolation period does not need to be extended, but the person with the new symptoms has to stay at home for 7 days. The 14-day household-isolation period will have greatly reduced the overall amount of infection the rest of the household could pass on, and it is not necessary to restart 14 days of isolation for the whole household. This will have provided a high level of community protection. Further isolation of members of this household will provide very little additional community protection.

At the end of the 14-day period, any family member who has not become unwell can leave household isolation.

C also could have had it but is asymptomatic.

Thanks @dementedpixie . Where is your quote from?

www.gov.uk/government/publications/covid-19-stay-at-home-guidance/stay-at-home-guidance-for-households-with-possible-coronavirus-covid-19-infection#ending-isolation

Thanks. At least I would be erring on the cautious side!