Converted House - communal electricity rising up in garden flat and raised ground floor Hall cupboards

As freeholder of a Victorian converted house divided in 1947 into 6 flats with communal electricity entering through and rising up from the garden flat to the main entrance hall raised ground floor cupboard containing 6 flat isolators and a recently installed fire alarm system for the building, my question relates to who should sign-off the installation of fire retardant cupboards and insulation between floors knowing that the garden flat is undergoing a full refurb with building control involved under the leaseholder’s instruction and in parallel the freeholder is having the raised ground floor main entrance hall cupboard brought up to regulatory fire safety standards and has instructed a contractor. The freeholder has been told by the property manager to use a BM Trada certified installer which has not been helpful.

Can the freeholder rely on the garden flat leaseholder’s building control sign-off of the fire proof cupboard installed by his builder following the regs or should the freeholder have both cupboards (at garden flat level and raised ground floor) installed by the same contractor and/or signed off by the same controller even if the contractors are different ?

Who would the freeholder go after in the event of a fire in the communal electrical cupboards spreading through the building?

If one controller for both cupboards would be allowed under the regs which type of controller should be instructed ?

I would greatly appreciate your guidance and suggestions.