If your varietie(s) are the taller, upright cultivars (those 14 inches to 2 feet tall) you need to seperate the roots and thin them after they have gone dormant in the fall or before they begin growing in the spring. This needs to be done about every two years or so. The lower growing types don't experience this issue. Stonecrop, as it's better known here in the U.S., makes a grand showing in late summer on into fall, surviving even the first light frosts...
I grow one of the taller border perenials (sedum spectabile) and I can testify they do tend to sprawl around if not given support, which can be a problem if they're close to a path or near the edge of a lawn.
You can try tying up the stems using canes and twine but unless your really good at it they can end up looking like corn stooks and more ugly than they were before, as an alternative to this you can buy ready made plant supports from garden outlets that are well disguised once the stems have grown through them. they are shaped like a hoop on legs which are placed over the crowns earlier in the year and the stems simply grow up through the mesh of wire.
For the late spash of colour they givebetween aug- oct and an important nectar supply for butterflies, a little effort is well rewarded. Good Luck. Tbird+