They are from the Iris family, a native of South Africa, and called Schizostylis coccinea or more commonly the Kaffir Lily. This red variety is called 'Major', and as you can see it has delightful red flowers at this time of year, rising above green foliage. There are other varieties too, in ranges of pink, but for me this is the one to have.
My surprise is not that the Schizostylis is in the garden, I did plant it there during the border planting in the Spring. The surprise is that it has flowered this year - it was a plant I'd had knocking around in a small pot for a couple of years - having procrastinated on where to plant it. At the time of planting I figured the flowers would complement the golden leaved Fuchsia 'Genii' beside it and extend the flowering period of the border, which also has Spring flowering bulbs, early an Summer flowering Geranium and some nice other plants too.
So here it is in flower 6 months later - great!
It is a plant that requires a reasonably heavy (we have plenty of this!) enriched soil, in a reasonably bright sunny position. It is a plant that benefits from regularly lifting and dividing (this also stops it from spreading too much)
so there'll be plants to spare soon to put into other beds and borders.
Do look out for it in your local garden centre or nursery.