YES, it needs fixing, that's gas you're paying for and getting no benefit from and could possibly be a hazard to yourself and others.
The reason for the pressure drop in your pipework needs to be identified and fixed, whether it be the pipe leaking or a joint leaking.
If the pipe was going to be replaced then whoever does that would have to expose the route the existing pipe runs to install the new pipe, so it would make sense, once it's confirmed beyond doubt that the pipe between the meter and flat is the problem, to expose the existing and trace the exact location of the leak.
There may be a joint that can remade, it may be a problem with lack of support for the pipe and had failed in some way which may also be an issue with the other flats.
What I wouldn't want as a customer is for someone to say it's quite likely to be the pipe, as opposed to saying the leak is at a specific location and pay for replacement only to find it wasn't needed.
As to whether the Freeholder has liability, that depends on the wording of the lease.
However it's quite likely you'll need the permission of the freeholder to have work done in the common area and the other flat owners need to be made aware of the issue in case it affects them or incase they have any knowledge of similar issues with their gas supply.