this plant grows wild and freely out of the walls in my garden.

What type of plant is this Tref?

I don't Know gilli, it grows like mad out of the dry stone walls in my garden.

Strange....it looks to have a woody stem....almost like a mini rhododendron.

To be quite honest it's a flaming nuisance, it's pushing my dry stone walls apart. the roots work their way between the bricks and expand causing the wall to gradually topple bit by bit.

Sounds like a job for 'Roundup'.....not that I like using herbicides but if it is destroying your wall I would say that it is the best way to go.