Ola Cabs / Ani Technologies — Charged more than quoted

I was quoted £2-4 for a 10 minute ride in London with a 50% off voucher. Once I was dropped off, I was charged £12.86. I have called to complain and they claim I accepted a quote of £11-15 which I absolutely did not. There are a large number of people who have also complained about this issue online and I believe they are running a scam or there is a bug. Needless to say, I am very unhappy and I would like a refund for the overcharge.