Assuming the original query refers to the two branches diverging south of Corby station, rather than Corby - Manton, and all with the disclaimer that it's been nearly three years since I had any involvement with any of it...
There's paths in the system for two round trips from Margam each day (steel coils to Corby). The morning arrival at Corby ran pretty much daily; the mid-afternoon arrival much less frequently (to suit demand presumably).
The automotive branch was blocked by fencing, a very short distance onto the branch. I discovered this shortly after I very nearly used said branch to hide an errant tamper, early one morning.
The sidings between those two branches and the single line (as was) towards Kettering were physically disconnected.
The sidings north of Corby station were regularly used as required to recess London services in the evening when turnaround times increased to ~40 minutes, as well as a booked move in the morning for the Kettering - Corby shuttle to hide before forming the 0926 to Derby.