Thanks for that.
"where this happens it is the homeowners' responsibility to pay for the repairs"
In normal usage if a radiator sprung a leak you would pay for the plumbing repair and the home insurance the carpet etc, would that not be the case here?
"as the Vaillant heat exchanger is notoriously almost impossible to powerflush properly"
But if I am reading you right a powerflush would be recommended and if it clears it, great but if not it could be new boiler time which would entail a PF anyway.
Possibly, it depends on the terms of your individual insurance. But you're correct in assuming that a powerflush is going to be needed whether you need a new boiler or not, so it's worth getting that done anyway, and also checking that your pump is operating correctly.