I wouldn't be surprised if the flue was not fitted properly in first place and the condensate has run down inside and caused the rusting, I doubt Ariston can be held responsible, they may however help/advise you (I have not had any dealings directly with them)
I am sure this is totally correct.
Its not the manufacturer's fault. They did not install the boiler and all their other boilers are working fine.
NOW the important part! You need to get a VERY competent boiler engineer to inspect it and that must be someone competent to make up a formal report with photos. It will not be cheap either, perhaps £300-£600. And it has to be someone who would be plausible in Court. You will have to pay for that at this stage.
You could free of charge complain to Gas Safe ( was CORGI ) but they will not be a witness in Court and my experience of their inspectors is that they may well be hesitant to apportion blame.
I would suggest you start by a formal letter to the installer headed "Letter before Action" explaining that the boiler has been found to have damage caused by an installation fault and you are holding him liable for the cost of repairing/replacing. You should give him the opportunity to put things right quietly and without further costs.
In reality the installer who does that kind of thing is possibly a cowboy and may not be registered or have any insurance. Easy way to check, did he compete the Benchmark Certificate and/or notify the installation to CORGI ( at the time ) now Gas Safe. Gas Safe should be able to advise you if it was registered.
You have not mentioned the installer. Either you bought the property after the boiler was installed or you already know that he was dodgy !
Please tell us the truth about the history and we can best advise you.
Tony