persons who we need to serve notice (they are occupying flat which they have made into 2 flats) have seperate doors but no door numbers

also logically they are 5th floor but have made it 6th in some places. neigbours door number is there - can we say ‘opposite 501, 6th floor?’ luckily they are on top flooor.

what is process to serve notice - one registered AD already came back when we wrote number that is in legal records and correct floor (5th)

can we get someone to go there and paste it? can only post man do this? can we hire a lawyer to do so? how much would it cost? thank you