Thanks everyone, this is really useful. When I saw how quickly it was growing in the full sun (and so soon after I brought it home) I panicked and hoped a bit less sunlight would slow it down! But you're exactly right, the room was still extremely warm. Not any more though! Would it be possible to r...

I bought a small Mammillaria Schwarzii in about March 2018 - at the time it looked like this: M_Schwarzii_1.jpg I had it on a south-facing windowsill and it grew very quickly. I tried moving it to a less sunny spot on a west-facing windowsill but it continued to grow regardless. The flowers died off...