So I've checked how all the bulbs are seated and the sockets all seem to be grounded ok (surely the indicators wouldn't work either if they weren't?)
And now mysteriously the passenger side stopped and the drivers side works periodically when i push the indicator arm all the way up (turning right).
Could the issue lie in the steering column with the wiring harness; are these even connected somehow?
I got a replacement turn signal switch from Classic Industries (that comes with the wiring loom) installed and now I have brake lights again and good turn signals!
However, now the horn constantly goes! I've had to disconnect (at the horn) for now.
Any thoughts on why?
I followed the workshop manual closely and pretty sure I've got everything in correctly however I know it happens once the center part of the turn signal switch goes over the steering shaft.
So, the diagnosis was spot on; the power wire had been pinched and was making contact.
I compared the path of the wire on the new switch to the old and they were different.
After taping up the cut I followed the path of the old wire and everything is now fully functioning.
To save others from this headache, attached is a photo of the horn power wire path from my old switch.
Thanks for all the help