You are quite correct about their requirements, Penny, protect them from winter wet. They do not like to sit with water in the crown as it will cause them to rot. The traditional method is to use a sheet of glass over them, make sure that the glass is not sitting on the lewisia and fasten it down somehow so the wind does not blow it away. If you can plant it sideways on in a crack in a wall that will also help.
Lewisia have a tap root so cannot be divided. Seed is the best method, but keep them alive first :-)

I grew Lewisia two years ago for the first time and they overwintered really well in all that bad weather. I left them planted in original container (part of a wooden train given to my hubbie) I tilted the container half on its side tucked up among other pots of plants near the house and they were absolutley beautiful this year and have flowered twice this season. I cut off the first flowers when they were over and they flowered again. Fab colour mix too.

Thank you both for these tips. I will do my best to keep them alive!

Most species are native to dry slopes in the Sierra Nevada, or the east side of the Cascade Range, in the Pacific Northwest of the States. L. cotyledon, however, is native to the west side of the Cascades, where it still grows in scree, but gets a good deal more rainfall. It--and many of its hybrids--could probably grow under ordinary rockery conditions in the UK, but most of the other species would probably need an alpine greenhouse to do their best. Those taproots are generally tuberous, so don't be surprised if the chink you plant it in gets wider, year by year!

Thanks for that information Tug! I'm planning to plant them on a roughly made slope of stones and gravel and for this winter at least, to make a roof of glass to keep them on the dry side.