It should be fine from the incoming supply that spits to the flats, as all flats are then on the same source and earthing arrangement anyway.
Check the bonding size though, as it would be determined in the PME case by the size of the incomer cable, not the size of the flat's feeders (see Table 54.8 in the regs).
For a couple of flats I would doubt the supply is more than 35mm (10mm bond), and very unlikely to be more than 50mm (16mm bond), so going with 16mm bond is probably OK, but check as far as you can as 10mm is cheaper and far easier to wrangle.
As already mentioned, ideally you don't want any joints in the bonding cable but if necessary then you can use something maintenance free like a proper crimp or soldered joint. Getting 16mm wrapped round your typical pipe clamp tag is difficult, you might need to have a longer screw/bolt than normally comes with them.