Europcar nightmare

I used Booking.com to book a car with Europcar for our 7 day road trip in the UK. This cost £118.23 (approximately R2163) We picked up the car in Oxford and dropped it off in ****stone, Kent. I authorized an amount of £344 on my Lloyds bank card to cover the £258.70 extra charges for licences, fees, insurance, drop off in a different location and an additional driver. On my return to SA I found that I had been debited by the full £344 and an additional £746.51. There have been over 25 emails, countless phone calls and I went personally back to the Europcar Oxford branch on 10 September and spoke to the Manager (Aaron Gunning) who promised to sort the problem out as we had been incorrectly charged for 5,000 extra miles (we had done 687miles), 2 extra days rental as well as 2 extra days premium cover. Not a word from him. To date I have had £799.59 credited and a further £290.92 debited. There is a total amount of £323.14 owed. Each time I call the customer service hotline - as instructed - I am told that they don't have the 'authority' to discuss this account. I have just phoned again, from home in SA to be told the same thing.