Fashioned an impromptu cat basket from a convenient cardboard box* and a blanket, and left the cat in the kitchen with the window open last night. When I got up this morning, the cat had gone. Aside from the foul weather, "jolly good" thought I. Better that she tries to make her way home. We were part way through breakfast when I heard some insistent meowing from the front door. Our waif (still as yet unnamed, because that would suggest that a) we had a right to name her and b) we were intending to take custody - she remains simply The Cat) was on our doormat, and had been cornered by two of next door's cats. Let her in, fed her and let the garklet stroke her.
We've left her in the kitchen again this morning; ias was planning to take a half day today, so she's going to try and get the cat to the vet to she if she's been chipped. If she hasn't, the next plan is to plaster the local area with "found cat!" posters tomorrow.
* this would be the box from the new 15" MacBook Pro that turned up yesterday - does this count as spoiling the cat?