Your other alternative is to scrape away all the earth from the suckers and then pull/tear them off and paint weedkiller on to each tear. It's a long fiddly job and will need doing more than once probably but that's the only way I found which eventually worked. Best of luck
The only othe thing you can try is to cover with a thick impermeable layer, and leave it for a least 2 years. If you do this after your last attack with the weedkiller, excluding all light may weaken it enough.
Could I go one step further than your suggestion @Allotment Boy and dig the patch down about 10 inches, rip out all of the suckers and then lay an impermeable layer in the soil. Then put new soil on top and reseed with grass seeds?
Do you think this would stop it growing or would the suckers just go around the layer and start popping up elsewhere?
Also would the layer cause issues with the grass on top?
Posts
Best of luck