Why dont you refer him to this web site? then he can see what we are all saying. I know it's often difficult to convince some people that BG are not the experts in this industry any more, also there is a danger of you not getting a job if your price is only one third of BG's price, customers can think they will get an inferior job when the difference is that much. Why not say you will do the job and upgrade his system to a two pipe for the BG price, that might work LOL. But back to your question: cant see that the "magnaclean" can do any harm, I would fit it on the boiler heating return.
Martin.