It's a type of sedum rockery plant. It will form a lovely green carpet! I have this too! If you want it elsewhere for ground cover in your garden, snip a stem off and plant the end in the soil. It takes well and looks great all year round :)

Don't panic , its not horsetail. There's a lot of confusion between marestail and horsetail and the names seem to be used interchangeably although they actually belong to different families. - the mares tail strictly speaking is a pond plant. Anyway the good news is that your plant is neither - sorry I know it well but its name escapes me.

Thank you both. I thought it looked like a rock plant but after the recent mare's tail post I wasn't as sure as I would have been. It's quite nice so I think I shall allow it to stay and create a green roof - as long as it doesn't cause leaks.

It shouldn't cause any leeks it only has small roots ( mine only has very shallow roots in the gravel) mine has spread very very fast allover the gravel but it looks fab!

Oops Dan, you beat me to it. Yes , sedum - I feel silly now.

I agree it's a Sedum Stone Crop. Looks very much like Sedum Rupestre 'Angelina.' It makes a nice ground cover.

I agree with the others, for clarification; marestail is an aquatic weed botanical name Hippuris vulgaris, and horsetail which grows on land is; Equisetum arvense, Derek.

I have the mares-tail in the pond! It's kept in check by me (if the fish don't get the leggy parts first)

Dan, my Mum told me when I was young that the mares tail is a miniature version of the large plants that are now coal.