Blackberry's

blackberries are persistent beggars you really do need to get all the roots out.

If you're left with regrowth you can't get out, the only way to deal with it is to poison it with SBK - try to use it on the bigger, lumpier bits of root, drill or cut into the root and apply the SBK (click on my icon and select blogs to see my blog on tree stump treatments - this applies to blackberry roots too and gives dos and don'ts, SBK dodgy stuff). I usually put a flower pot upside down over the treated bit of root (because it's usually below the soil level) and then replace the soil around it - means you can find it again if you need to retreat and avoids you planting where you've just poisoned. I now no longer bother to attempt to dig out blackberry roots, just cut back to the lumpy bit and treat it, it being easier to treat a larger piece of root than a tiny one.

There will be lots of seed stored in teh soil - you just need to remain vigilent and dig them out as they appear

That't the easy bit, Sid, new seedlings!