It comes down to who is the 'responsible person', if the developer owns the building and has sold the flats on a leasehold basis then it will be the owner who is the responsible person for the common parts.
however if the 'owner' has appointed a managing agents and by virtue of their contract is responsible for safety matters then it would be them that would be responsible for the Risk Assesment.
Article 5
Duties under this Order
(2) Where the premises are not a workplace, the responsible person must ensure that any duty imposed by articles 8 to 22 or by regulations made under article 24 is complied with in respect of those premises, so far as the requirements relate to matters within his control.
(3) Any duty imposed by articles 8 to 22 or by regulations made under article 24 on the responsible person in respect of premises shall also be imposed on every person, other than the responsible person referred to in paragraphs (1) and (2), who has, to any extent, control of those premises so far as the requirements relate to matters within his control.
(4) Where a person has, by virtue of any contract or tenancy, an obligation of any extent in relation to—
(a) the maintenance or repair of any premises, including anything in or on premises; or
(b) the safety of any premises,
that person is to be treated, for the purposes of paragraph (3), as being a person who has control of the premises to the extent that his obligation so extends.
(