Help deriving a 2-port representation of an RC filter circuit?
Can anyone provide any insight, even if its just to point me in the
  right direction


Here's a clue how to deal with a simple resistive potential divider: -



You will need to use C1 in place of R2 above but remember it's C1's impedance that replaces R2.

\$R_S\$ can be lumped in series with my R1 above.

CL can be lumped in parallel with your C1.

Can you take it from here?


  I know that 1/(jωRC) gives the impedance of the capacitors but I don't
  understand how to make that into a real number.


You can't.