On closer inspection, it appears that I have a tray with the necessary groove. However, having used the ‘chunky’ SOC brackets to mount the tray, they do not leave sufficient room for the rod to run in the groove.
I could. of course, grind off a section of the inner half of the bracket but, as the brake arm is moving slightly in the brake plate (obviously a bit worn and requires a new bush), I will pull the lever unit out and extend it by an inch and fit a new bush.
Once I have the rod running free, perhaps new springs internally will ensure that the arm returns to the correct position after use.