Hello Kentish Towner and I hope you get to see this.
I was checking the thread before promoting it on social media, and spotted that your original Q. never got a reply to the John Innes part.
It is not a suitable filler, too expensive for what it brings. You are better with compost than soil. Compost can be anything decomposed, including old animal manure. It sinks while your plants are growing, then add more after a year, etc.