Have you tried a systemic weedkiller like Roundup? Or a brushwood killer? I know that digging it out is difficult, but it should work if you keep at it where you can, and use chemicals on it where you can't dig.

To get rid of spreading bamboo in garden is a big challenge.
It is recommended to make plastic or iron border around roots at the time it is planted. This barrier should be at least 50 cm deep in soil. Otherwise will happen what happened to you. Bamboo is spreading by roots.

I'd dig a 70 cm deep trench all the way around where I want the bamboo to stay, and line it with concrete, or 32 mil (I don't know the metric equivalent) or thicker plastic. Next, outside the barrier, I'd dig up as much of the canes and attached roots as I could, cut off the rest of the canes, and IMMEDIATELY paint the cut ends with concentrated Round-Up--if I waited 10 minutes, the vessels in the stem would seal up. I'd then watch for sprouts, and cut or break them off as soon as I could get hold of them, and again IMMEDIATELY paint the cut ends. That should get rid of the unwanted bamboo in about 2-3 years, though it might take longer. Whew! A lot of work, I'll admit!

Many thanks to the 3 responses to my question - its sounds like I am going to have a lot of digging and will have to be patient. Thanks akd

Tell me about it Akd, I am currently removing a wayward Pseudosasa!