Opodo flight overcharge

Hi. I booked a flight through Opodo, out from Heathrow to Boston, and back from JFK to Heathrow. I searched Kayak and was linked to Opodo through my chosen flight and price. This price was replicated on the Opodo website and was throughout the booking process. The total price for 2 travellers was £1530, and I had to confirm my acceptance of that price when I paid. When I checked my confirmation, I had been charged a total of £3,400. It appears the outbound flight has been booked as Premium Economy hence the price difference. I am certain that the price I agreed on the website was £1530. Clearly, however, I don't know how I can prove that. I re-traced my steps the following day through the same websites and flights (recording what I was doing) and again the price was £1530. A closer look at the detail showed that baggage allowance was larger (matching Premium Economy allocation) and the seat selection was only allowed in the Premium Economy area of that aircraft. There are no Premium Economy flights with Virgin available anywhere near the price that was being quoted. Opodo to date have refused to accept their error. I am continuing through the process of complaint. However, I travel in less than 2 weeks and if they accept their error and cancel I will need to book another flight.

In this instance is this an example of theft? What are my rights here? How can I prove a technical error on their part? Would this stand any chance of standing up in a small claims court? Any help gratefully accepted. Thanks.