It turns out that I spoke too soon regarding the large moth and that it had in fact remained inside the house (though I can't imagine where). It showed up in the bathroom this morning and was very relieved to be let out after being photographed.
To the naked eye and with the flash off it looked a very dull brown - similar to a ringlet butterfly, but the flash revealed quite a distinctive pattern. It's the largest moth I've seen here in Devon and with wings spread was the size of a peacock butterfly. All suggestions as to the species are most welcome.