I did grow Saxifraga x arendsii 'Flower Carpet' from seed a few years back. It was a cheap pack of Sarah Raven seeds that I bought in a seed sale on her website.
They were successful in that I got a lot of plants and they weren’t that tricky, but they are slow to get going. I just followed the pack instructions, which I don’t remember exactly, but they have to be on the surface of the free draining compost just covered with sand and I think sown in autumn to overwinter and break dormancy.
We had a few bad frosts the following winter and I lost the lot unfortunately. Perhaps because they were only young plants and could have done with going into the cold greenhouse. If you want loads for putting into a wall it is a cheap way to do it though.