2 out of 200, with 30 actually going caving to begin with, sounds pretty good. Certainly above average for the country population:caver ratio. I don't suppose you happen to know approximately how many trips per year the remaining 2 cavers were doing? Do you have a record of the drop off rate and after which trips people began to dwindle? - i.e. was there a nasty cold tiring trip in February attended by, say, 9 people after which only 1 person turned up to the next outing? If so, it should be possible to tailor your meets list to minimise the likelihood of scaring people away too early by holding back on the gnarly stuff a bit longer. Just a thought.