On another note, I have £10m public liability cover whilst scuba diving covered via my BSAC membership (£65.50/yr). Apples and oranges (I would struggle to damage a Citation with a snorkel), but simply using this as an example that it should not be overly expensive to get sufficient cover.
Well, like you said, it's pretty hard to damage a Citation with a snorkel, and insurance companies know this. So getting public liability cover whilst scuba diving will be relatively cheap. But it's much easier to damage a Citation with a car, and insurance companies know this too. So getting coverage for you car will be a lot more expensive too.
I'm wondering about something else though. A lot of airfields require visiting aircraft to have liability cover to a certain amount. Should they not also requires something similar for private cars allowed airside? Or is this possibly, maybe, included in the airfields general insurance?
Edited: I checked my (Dutch) car insurance and there's no exclusion in there for airfields or anything else that's not a public road.