Plant ID: can you help where the web has failed me? What type of Euphorbia?

Along with a pot I got from a neighbour who recently moving, I've inherited a plant that I'd assumed was a euphorbia.

The particular specimens in my pot have clusters tiny leaves, only a couple of millimetres across on a plant with a rather floppy habit about 20 centimetres in height.

I'd originally identified it as the freely seeding petty spurge (Euphorbia peplus). But that seems to have a more open structure and it's leaves are clearly bigger. Below is one that arrived in my pot of Ruby Beauty Raspberry which had a very bad spring. (Apparently the little loved peplus can be used to cure melanmoa!)

I can't find anything that matches the small leaved plant pictured in the first two photos. The neighbour wasn't much of a gardener. So, I suspect the plant will either have come from a garden centre or arrived on the wind.