Such a good option Iain. Inserting the cat-flap into brickwork is more complicated but very safe. Cat-flaps can be used by burglars to gain entry into your house if fitted to a door or window.
I don't have a catflap myself....Our cats have their own private doorman in the guise of Angel, my dog. Whenever they stand at the door (back door, I hasten to add) looking out, she just obliges by jumping up and opening it for them. There isn't a door in the house that she cannot open more quickly than me. Drives me bonkers at this time of the year!!!
Jenn, there is a product on the market called 'KatKabin'. It is a small, completely waterproof, off the ground, 'kennel' type affair that has been designed for cats. It would be ideal for just the scenario that you envisage....a warm safe haven for a cat to use until their owner comes home. I think it retails at about £40 - £50 but it is a beautiful product, and something on the lines that we had been considering introducing for some time, but just didn't get around to it.
Hope that helps....I'll try to look up the link in a mo.
Best wishes,
Roz