No 10 says Starmer 'inaccurately + selectively' quoted the March 12th guidance - see it in full for yourself here - https://www.gov.uk/government/publications/guidance-for-social-or-community-care-and-residential-settings-on-covid-19/guidance-for-social-or-community-care-and-residential-settings-on-covid-19#guidance-on-facemasks

The document which was published Feb 25th then withdrawn on Mar 13th says twice that it is very unlikely people in care homes would get infected because there was no community transmission of the virus at that stage