Try:
Brown from feed to COM, brown to bathroom to L1
Brown from other feed to L1, brown from 2 core and earth for hall to L2, 3 core to hall brown in L1, Black in COM and grey in L2, depending on how the other switch was wired
Do the porch switch the same as the other one in the circuit, probably brown to L1, Black to COM, grey to L2.
There are several ways to wire 2 way switches, but this is how I would do it.
Adam
August 2009