2020 Premium Increase Shock

I received a notification of an increase in my premium which works out to 26%! I contacted the call centre seeking an explanation as to how this was calculated but the agent keeps repeating the content of the e-mail and when I asked if I could speak to someone to get more detail and clarity I was told that anyone else that I speak to will tell me the same thing as the agent just did. I think it is a perfectly reasonable request to seek detailed information to justify such a large increase. This is a complete turnaround from the amazing service I received when I joined Oneplan. I am disappointed and feel maybe I would be better served changing insurers.