In a criminal case the guilt has to be proved beyond all reasonable doubt.
That does not apply to civil claims! They have to be proved just to the balance of probability.
However, information provided above changes the situation!
I, and everyone else, had not previously heard of any inherent defect on that model before. Assuming thats true, which I am happy to accept, then the boiler does apparently have a defect.
However, that does not give rise to a good claim against Ariston because the householder failed to have the boiler serviced as required at the end of the first year. If that had been done then the early signs of the problem could have been identified and deat with.
In the revised circumstances you should contact Ariston, in writing, stating that their boiler obviously has a design fault and you expect them to deal with it.
To persue a claim against them you will need substanciated evidence in writing. "How Much longer" may be able to help you here. An anomous posting on an internet forum does NOT constitute evidence though.
Hailshams forthcoming advice to persue the installer is questionable given that, in good faith, he fitted a boiler supplied by a reputable manufacturer. He fully expected you would have had it serviced at the end of the first year and any defects would have been dealt with at that time. I still say persue Ariston! They have a warehouse full of boilers and spare parts and could easily give you whatever is needed to sort it out. The installer is wondering where his next meal is coming from.
Tony