Very costly benefits

Hi there,

My previous employer in the UK provided me with a company car and told me not to pay for my private mileage.
I much later found out that all of this would be extremely costly for me because of the car model and because of the private mileage benefit. I was not warned at all by my employer about this and am not from the UK so had no idea how this worked, i.e. that the benefit cost was linked to the car model and CO2 consumption nor that the private mileage had nothing to do with the actual mileage but again was linked to the car model...
I barely used the car for personal use and when I did, I paid for the gas myself.
I am now asked for a lot of money by Revenues and Customs since they apparently made calculation errors in the past, but I don't want to pay for a benefit that I did not enjoy. I have contacted my previous employer to ask to reconsider this benefit in my P11Ds but have had no reply at all.
What can I do? I'm a bit at a loss here but don't think it's right to pay for something I did not benefit from and which was never explained to me.
Thanks for all your help in advance! Much appreciated.