Help needed in identifying mystery shrub.

I have been out in the garden today restoring order, including cutting back and extraordinary tangle of dead branches and shoots muddled up with the rambler I am retying to its supports.

It has stout pithy stems, reminiscent of elder, mid-green leaves, some of which have survived the winter, and in October, when I moved in, it still had loose sprays of small white flowers.

It's multi-stemmed and is coming up in the border, from INSIDE the wall it's growing against and appears to be shooting from a buried stem almost like a runner. Most extraordinary is the way it's roted itself into the wall where there are loose stones, about seven feet up. 

Any ideas?