I've done nested flights from the UK to LIS for ages- I ususally get a full fare ticket from LIS to the UK at the end of the Christmas holidays and usually use it to go back home the following Christmas, but it's available to get back there at short notice in an emergency. In between I'll go back to visit the in-laws a couple of times or, for almost a year when I was working on rotation, every 2 weeks. No-one's complained yet....