I know this is received wisdom, but I got a really decent sized and aggressive growing clump out in under a day using an angle grinder and enthusiastic mattock-wielding.
Done similar for next door (and got a load of huge canes out of it as a bonus - I have a strange American made chainsaw disc for the angle grinder). Really not convinced it won’t rise from the dead though.
I know this is received wisdom, but I got a really decent sized and aggressive growing clump out in under a day using an angle grinder and enthusiastic mattock-wielding.