Unlike in the USA where, and i cannot understand the logic, many ducts are placed in the attic, I suspect the old ducts in the UK stay within the building envelope. If there can be a 10C difference between the air temp at the lowest level and that of the attic, he can transfer heat with just the 2 hydronic coils and a small pump. The piping would be 3/4" PEX or whatever is available. I think it could work but it depends on the attic temps.
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