How do I renew my China Travel Permit(passport) for overseas Chinese originall born in Hong Kong?

My China passport/ Permit issued by the China Embassy in London has expired this year during the Covid virus restriction period.
It was previously renewed in London at the expiry dates. However, several visits recently to the Embassy during weekend mornings found the basement office at Portland Place padlocked and inaccessible, despite a sign posted there saying it is open weekday mornings. .
Please advise how do I get to get my document renewed.