The internets tell me that the Hydrangeas of Insanity are in fact Annabelle hydrangeas. This is awesome: it means that even if I want to keep them, I can prune them right down the ground, because they bloom on new growth. So, armed with a nice new pair of stainless steel pruning shears from Lee Valley, I plan to muster out there this weekend to attempt it. Depending on how big they are at the base, I may leave them to regrow - since they will undoubtedly be smaller than they are now - or I may dig some of them up to make more room. But in any case it will certainly be less labour-intensive than digging up entire massive overgrown shrubs.