What are the most reliable air to water heat pumps?

[HMK]

I want personal and professional opinions on what manufacturers supply the most reliable air to water heat pumps.
I am asking this because my air to water heat pump has failed, installed a little over 6 years ago, the cost of repair is most likely uneconomical as the heat exchanger has leaked and all the f-gas has gone, a replacement heat exchanger for my make and model of heat pump is apparently as rare as hen's teeth.
I will not mention the manufacturer yet as I am still in discussions, as although the warranty has apparently expired current equal models by the same manufacturer offer a seven year warranty. I am arguing that the product they supplied has failed to meet its expected durability under UK consumer legislation.
At this stage I am only interested in feedback on reliability of product as I am a pensioner and do not want to have to be finding money to replace the replacement air to water heat pump again in less than 10 years time. When I had the current now failed unit installed I was assured that it would last for at least 10 to 12 years, and many articles give opinions that suggest an air to water heat pump should have a life expectancy of well over 20 years.