so what if it's a little old lady without the means to pay for a boiler upgrade......is she just supposed to freeze for the winter .....can't help wondering if this would happen in Germany......ffs......!!
sorry didn't see pic.......they have created the problem so in a perfect world they may be expected to contribute to the rectification works......in a perfect world.
you need to start with their builder ....if he won't sort it ask who drew the plans and signed off on them .....pretty sure the issue should have been highlighted during the planning stage , if the engineer has deemed it unsafe / illegal then someone hasn't done their job .