Aztec,
Very good question, who has got a concept o2 system?
There used to be a sticky post at the top of the ciu forum about this. Mike Boxall the CIU sponsor and express cleaning owner has some kind of tie in so they are a highly reputable company.
A lot/(some) of people think their hot system is the best that money can buy. I think that, but I am not an expert.
From what I understand the hot system includes an LPG type tank conversion to feed the heater. If you had this done yourself it would be expensive, and there are exacting requirments to meet.
I believe that an infinity26i with a powered flue is used to heat the water. This is state of the art with a digital thermostat.
You get what you pay for. This is the best.
But.... your question stands, who has got a concepto2 system?