Thanks BadLad that's good info. We actually did the trip yesterday evening. The weather had quietened down, but there was a lot of water on the fell and the stream into Lost John's was high. It's a Cracker was a cracker! and although the water was obviously highish, it was not flood conditions. Our assessment (before reading BadLad's post) was that you probably wouldn't want to be there if the water was much higher. It is a wet place, both on the pitches and between them
Talking to the guy in the Whoop afterwards, he said they'd had a huge amount of rain in the past 24 hours, but as BadLad points out, the cave sits high in the catchment area so it will drain quickly as well as flooding quickly.