I seem to remember that you cannot combine swa & core, as form of earthing or bonding, but I might be wrong. What was the exact reading you obtained for suspect extraneous conductive parts?
GN8 mentions this but it refers to PME
Just a quick excerpt from Guidance Note 8 on this:
For installations forming part of a TN-C-S system in which the supply is earthed at
multiple points (PME conditions applying), and it is intended to utilise the armouring of
a cable or a core of an armoured cable as a protective bonding conductor the designer
needs to consider the effects of the currents that may flow in the conductor due to
network conditions. The armouring or a core of such a cable is normally not used as
a protective equipotential bonding conductor where PME conditions apply unless the
electrical installation designer determines that the heat produced in the armouring or
core due to its use will not cause overheating of the live conductors of the cable when
on full load.
Then there is another passage on combined armour/cores:
9.3.4 Armouring inadequate for a CPC
There will be occasions where the actual CSA of the armouring of a cable is less than
the value of S calculated by use of Equation (9.1). To remedy this, the designer has
three options to consider:
1 selecting a cable with an extra internal core, and using it as the CPC (note that all
the cores are required to be suitably identified – Regulation 514.3.1 refers)
2 selecting a larger cable with a corresponding larger CSA and using the armour as
the CPC
3 a separate green-and-yellow covered copper conductor.
For options 1 and 3, it cannot be accurately predicted how the current will divide
between what is effectively two parallel conductors (i.e. the core or separate conductor
and the armouring) due to the magnetic effect of the armouring. It is therefore
important that the additional core or separate CPC is sized as if it alone were to take
the earth fault current. In other words, it is not permissible to add the CSAs of the two
conductors together.