advice on insurance rights

fatr1ch1e

In October last year my daughter purchased her first car insurance policy via adrian flux. At the time she estimated an annual mileage of 5k and took the policy on monthly payments. As she was under 18 at the time i signed for the credit on the monthly payments. In February however she had almost used all her mileage as she has been travelling up and down the country for weekends away. We called adrian flux who quoted roughly an extra 400 pound for 10 k miles instead of 5k. They advised it would be split over the remaining 4 payments. Sure enough a letter came from premium credit advising the same. 4 payments of roughly 100 more a month than previous. This month my daughter decided to change cars. She got a quote from adrian flux who advised an extra 168 on top of current premium for rest of year. We had 2 payments left to make on the current policy of roughly 283 pounds. They said they will add the increase on and then.split the balance over 3 months so payments would be 249 ish. This seemed very reasonable and an affordable option for my daughter so she bought a different car and called back to make the changes. During which they confirmed 3 payments of 249 ish. I have now recieved a letter from.premium credit stating 3 of 340ish to pay. Obviously way more than expected. They wont change the payment as its down to adrian flux. Adrian flux say that they miscalculated on the first call about the car swap and hadnt included the additional mileage increase. However what they quoted does add up correctly to the additional mileage increase payments. They wont back down and I am livid. My daughter simply cannot afford the extra 90ish a month on top of what they quoted and would not of swapped cars had she known. What are our rights here as surely its a missold policy if I have figures in writing from previous payments and 2 phone calls with them quoting the same increased premium. ??
Payment is due out my account in a couple of days and they are now conveniently taking 6 working days to respond. Previously it was 48 hours.