I do still have my spit.... maybe I should look at doing the underside myself...
Yep, go for it.
I painted my car on a spit. Spent ages on the underside paint to get is looking nearly as good as the topside, then covered it in body wax and never saw the paint again. MOT man was the only person apart from me to ever see the underside clean, and that was only once. Next time I'll concentrate on rust protection rather than finish.
More importantly, while the car is on a spit you can apply all the cavity protection you need and angle the car so the Waxoil or Dinatrol (my choice) runs downhill. It's a comforting feeling to see the cavity protection creeping out between flanges because the sills and other areas are flooded.