Hi Jamie,
I think I know what you need.
on my series 3 the drop down plate for the towball has the holes for the pintle at the top, but to fit it you need to remove the plate, bolt on the pintle and then bolt the plate back on using longer high tensile bolts and really thick washers (due to the bolt heads) however to use the ball you need the pinthe off. my fix was to weld the heads of the pintle bolts in place for easy removal of the pintle when i needed the ball.
Hope that made sense,
**A quick trip to the shed reveals that i drilled the mounting holes myself as my spare lacks the holes. so not entirely sure where i got the idea / reference**
Nick