Quote: "is there any way of, say, deleting the underside faces but leaving the vertices?"
I don't think you can, so your choices basically, are:
- Start it again
- Set all of the viewports to "smooth shaded", and disable "Draw Backfaces". Equip yourself with that fine old select tool. disable "Ignore backfaces" and "By Vertex". then everywhere you see a hole: select in the middle of where that triangle would be, and hit CTRL SHIFT F (check there are no other faces selected in other viewports!)
- If you've not regrouped the second half of the model, delete it for now... If the base faces are all part of the own group with nothing else in them, you'll have done yourself a huge favour, since you can just unselect everything, go to the groups tab, highlight the relevant group, click select, and again, CTRL SHIFT F