Having checked my girls about lunchtime, I think I have identified the culprit. As soon as the weather improves, I will put her in the sin bin (an old rabbit hutch I use for broodies) - I will keep you informed of results.

I don't have rats as far as I am aware, but I do (did?) have mice. I say did because on Monday, the chooks caught one as it ran across the chicken run and proceeded to rip it apart! Im sure if the other mices saw this they have either gone or have their bags packed!

The girls aren't currently in moult, but when they were in moult (at the end of last year) you're quite right, they were off lay for a week or so.

Thanks for the replies.