Tube in tube? Is that what the Intergas hex is?
You know that the Intergas is not tube in tube. The Ferroli is the one that is tube in tube.
Intergas filed their patent application in 2014. Ferroli filed their application in 2009 for tube in tube with the outer tube having an ovoid section
There is an Intergas application dated 2009 for an exchanger consisting of two separate tubes in a pressure moulded aluminium block.
There is an earlier Ferroli application which relates to tube in tube construction but where the outer tube has a circular cross section. No doubt the later change to an ovoid section increased the efficiency of the combustion to water exchange
From this it does apear that Ferroli might have been ahead of Intergas in the development of a combi boiler that did not need a secondary heat exchanger.
That said patents are worded to confuse the opposition while making sure the patent agents can understand and protect the "invention" that is being patented so the real and accurate history is difficult to obtain from even the most detailed search of patents.