Will this Cafetto cleaning tablets work with my Sage Dual Boiler espresso machine (the "by Heston" range)?
Yes, I use aftermarket cleaning tablets on my Infuser and Profitec. I haven't used the Cafetto branded ones but they're all essentially the same for backflushing.
Just follow the normal backflush regiment and use these tablets instead of the breville ones and you'll be fine. Personally I use Urnex Cafiza tablets, but again, they're essentially the same thing.