@msmithuk77 strange it seems like your plumber is 80% there with his knowledge, but a 35kw boiler is insane, an 18kw would do the job undoubtedly
I'd do as @JohnMo suggested and run everything at the same flow temperature, this will provide maximum efficiency, reduce the amount of equipment needed, reduce electricity (pumps etc) and also reduce the amount of things that can go wrong in the system.