The Bush Regeneration Team has been working on holly at Mt Wilson for many years now and has found that scraping/cutting and injecting, as pictured, is the only process that works with mature holly plants.
This specimen was treated at Mount Wilson. The most important aspect of the treatment is to remove the ground layer to expose the roots and give them a good scrape. Then every metre or so up the trunk needs to be scraped, as the plant compartmentalises. (This means that even if you have treated the base, the plant will continue growing up above.)
So if you see any plants out there treated like this, the Council Bush Regeneration Team has been there.