I looked at a few Semps on the RHS Lynn, it doesn't seem to give information on crosses but maybe there is where it can be verified, there's a lot of them! It does give crosses on Astilbe where info is available, and whether it's an accepted name. For Sempervivum alone there's 1,152 records listed, I know there will be more but it's at least a good place to verify a name.
