If you have a company car and/or any other taxable benefits the tax on those is your responsibility. Your former employer will also have a Class 1 A national insurance liability on the benefits which is their responsibility. They should have reported the fact that you had a company car to the tax office by completing a P46 (Car)
. Many employers are lazy and don't bother. Given that you were probably aware that the benefits you had were taxable and your net pay didn't change, you could have contacted the tax office yourself to have your tax code adjusted so it will be difficult to argue your way out of paying the tax. However, take a look at the notes here
and see if you can get the tax office to investigate your former employer. It's a long shot but you never know.
You can use the calculator here
to work out the taxable benefit for your car. However, you will need to know the original list price of the car and its CO2 emissions figure. That information can be found here
I'm afraid that as far as the medical scheme benefit is concerned, you will have to estimate this unless your former employer is more forthcoming with information than they have been to date.
I hope this helps but let me know if you have any further questions.