It's pretty much random, katlaughing. Doesn't seem to make any difference what time of day it is, central heating/water on or off, light or dark, even my singing doesn't stop it.

A friend has just come up with a brilliant idea though, which I may try this weekend. There's a highly inquisitive Patterdale terrier of our acquaintance, Peggy, not a mile away from here. Peggy is known for her unerring search-and-destroy capabilities. She usually destroys things which weren't remotely intended for destruction, it has to be said, but on this occasion I might just be inclined to invite her round and let her have her head.