I just thought I would update you all with what I have found and also what I have decided to do.
I found that the original Fastback harness is supposed to have 6 wires running through it. Mine has only 5 so straight away you know something isn’t right. I then decided that I had two options at this point; the first was to completely replace the whole harness with a reproduction one, or the second was to run a wire from a feed under the dash and wire up the lights myself.
A deciding factor was the fact that my next task is to get the reversing lights working and I want to know that the wiring is correct when I start that job. Also, in the interest of originality, I decided to get a whole new harness. At least this way I know everything should be in order from here on.