Ah, but this is an English back garden and the only body of water is a small garden pond so I think crayfish are rather unlikely ;)

The ground is hard and dry because the holes are in a raised banking and we've had hot weather and very little rain for the last three weeks.

The mystery continues, I think I've found another small hole and on of the neighbours cats was showing some interest. I'll just continue clearing ivy, brambles, and other undergrowth and see what else I find. I'll keep posting pictures as I take them.