I am surprised the insurance company have not discovered that many years ago you took time off work, suffering from flu and did not declare this to them, hence the insurance is void.
Aside from the above, if I were you, I would ask the insurance company if you had booked and paid for a hotel and had to cancel � would they pay out for your losses? Try this route, but I doubt it will work. They would no doubt argue that at worst they would pay any cancellation charges that might apply. Since you are not canceling the accommodation, there are none.
You could ask for advice from travel journalists, Gill Charlton of the Telegraph often answers such queries from readers of her column, and if she believes they have a justified complaint � fights their cause, often with a favourable outcome for the reader.