Honestly, no catch, Gardeners World do these offers every so often - I have had them and a friend has too and they are just what they say they are.
48 perennials for Free just pay cost of postage which is £5.65
Ideally you'll need to put them in pots to get to a decent size before planting out - but a real bargain if you do...............
(Thanks H for directing me to it)
Here's the link just click: Free offer