Your original question says:
"Layout consists of a single "main line", with a point coming off into a private siding, fairly standard 2 sidings and a small headshunt plan which I don't think needs reproducing here."
Given that you also state that the layout is set in the 1970s or 80s and in the south west, the simple answer is that there would NOT have been ANY signals linked to access to or departure from the siding.
Even with a signal box nearby, access to the siding would have been controlled by the mechanism used to control access to the single line. In this location and era, this would have most likely been an electric key token (EKT) although train circuit block (TCB) or even direction lever (DL) wouldn't have been impossible. If the signal box controlled the siding access then that would have been physically linked with the loop that the box controlled and not with the single line itself.
The point would have been worked by a traditional (and very simple) GWR/WR 2-lever ground frame. One lever painted blue would have been released either physically by the EKT or electrically from the box (TCB/DL) and would have worked the facing point lock, the other lever painted black would have worked the points themselves. There would have been NO fixed signals, the person working the ground frame giving the necessary hand signals to the loco crew.
This was a very standard arrangement on the former GWR, with examples dating back to at least the earliest years of the 20th century, and with the WR finally taking over the ex-LSWR lines in the south west in 1963, it rapidly became the standard arrangement on those (substantially rationalised) lines too.