weedol this is quite a good weedkiller

Roundup and digging. If you want to plant a garden afterwards don't use anything that will leave a residue in the soil.

After weedkilling your brambles let them die back and then cut off all the main branches down to about 6 inches. Getting down close loosen the soil around the stem and follow back some of the major roots (they can be several feet long). Cut these or pull them out. These weaken the hold of the plant and you can then rock it about and pull out the whole stem. 3 years ago I cleared a derelict plot one summer in this way and I now only have to keep an eye out for weak new growths which get pulled out easily.