What have I found?

I've just found a strange seedpod in my garden. It's a dark green ball about half an inch across. It looks like an unripe acorn in its cup, with a sort of indented circular bit with a tiny mark at the top, but it's soft and pithy. When I opened it, I found two knobbly seeds, with an opaque covering, and the seeds were grooved, like a brain. They look like some things I found last year, but they were in a cluster (I thought the birds had left them), and they were bright pink.

Anyone know what this might be? Some sort of shrub?