over charged for the past 5 years

i have a bond of 630000 with standard bank, when i enquired 2 years back regarding the insurance premium that is high, i was told they will look into it . now i received a letter again that they will increase my premium i then called again to find out as to why if the value of my property is less than 700000 but i m insured for *** ,still i cant get any assistance.

i m really frustrated now , because i feel like i m being double charged for my premiums. my house worth is not even close to 800000 yet i m being insured for over ***.