Background:
I have a portable TV which I want to add a headphone socket to.

Plan:
Open TV.
Find wires (probably just 2, I doubt it a 14", £80 TV has stereo)
Add chassis mounted switch to the live (?) wire.
Add chassis mounted headphone socket to the casing
Wire the socket to the other 'side' of the switch

This will (I hope) leave me with a TV which I can switch between
headphone and speaker modes.

Is this just for your personal tv? If so you are probably ok with the advice
from the other posts. But I do remember the technician at my university
language lab telling me that when he did the tv's there, he had to install
isolating transformers between the headphone socket and the speaker wires (I
guess due to the possibility of live chassis).