I've managed to adjust the spring with the new tool I bought. It didn't come with any brand name or product ID so I've taken a photo to give you an idea of what you need. I have a chat with the mechanic too, he said that the Honda tool wasn't hardened, so it wasn't going to last forever with a lot of use. He also said to prevent it from happening again, you should get all the weight off the back wheel by lifting it if possible (centre stand?), or at least removing the top box and unnecessary accessories and getting someone else to provide some lift!
Honda Tool (showing the rounded tip)
I've included a shot of the generic tool that won't work. Because you can't see what you're doing around the back of the spring it's impossible to get a grip, and the thick handle prevents it from turning when you do:
generic tool (won't work)
The below tool is what worked for me (it doesn't have a brand name). The right-hand (smaller) end was easiest, and the thinner handle meant more angle for turning.
Hardened tool