Like I have said...We await Vaillants decision . If they say they are happy to put a warrantee in place as there has been no warrantee in place legally (This is not open to debate its all in their terms ) Then fine . If they say there is an issue then we put all the cards on the table to The heating company and take it across social media if they dont want to step up and make amends . Something the new American owners of the company might not like ! And make people aware and check their installs to see if they have the same issue .As according to a "manager" its very routine to fit them on the flow of Vaillants . I have seen companies act far quicker when a issue is "Made public" than going down the legal path .At this point you need to put up or shut up, i.e. either sue the installers for whatever damages you feel entitled to or get over it and move on.
As someone with some experience of technical disputes I would recommend the second course of action. As lawyers are fond of saying, a bad settlement is better than a good case. Your case is weak unless you commission an expert report that quantifies your hypothetical damage to the heat exchanger. Such a report, assuming you can find someone willing to do the work, will cost considerably more than just replacing the HX on spec. Keep in mind that there's no guarantee that you'll get the costs involved back even if you end up winning the case.