We're in the process of renegotiating with the company so we'll see where that gets us!
In terms of communication, I'm not sure what else I could have done, I have a rather long email chain with them organising this and in every message the words power flush are mentioned somewhere, and as
they describe themselves as power flush specialists on their website, I would think they should know what I expected when I asked for a powerflush. There we're numerous opportunities for them to comment that a magnacleanse would be more suitable, but there was nothing of the sort to lead me to believe they would do anything other than what was quoted for.
I appreciate the comments with regards to the magnaclean We'll continue emptying it before our boiler is fitted next weekend, I just don't want to invalidate our warranty because it wasn't installed to the correct standards.