Discovery Insure -13,5% increase in premiums with annual review and they can't give a reason why!

I have been with Discovery Insure for a year now and have, until now been very happy with their service, benefits, etc. I have just received my annual review letter, and I am shocked that my total monthly premiums have been increased by 13,5%. I phoned them, and was told that they are sorry, but they cannot relook at the premiums because they have just reviewed, everything. So, as an example, I have 3 vehicles insured, and despite the values dropping through depreciation since last year, each vehicle's premium has gone up by around 16%. I had NO vehicle claims last year. This is really pathetic. I have virtually never claimed and feel **********.