Sometimes browsers or systems do hang, but unluckilly this is also the most common reason or "excuse" that customers give as well. Hence my suggestions to help you to next stage.
If you receive no reply then you need to go back to your credit card company with Opdos lack of response and reiterate your claim that no transaction was fully completed or confirmed and that Opodo are unwilling to provide proff that your claim is incorrect. You therefore hold the credit card company responsible for resolving it. Why your Unclue will have trouble is explaining why when he wasn't certain whether a booking was made or not - evidenced by the fact that he e-mailed - when he received no response he decided to assume based simply on the fact that the credit card transaction hadn't been processed - that no transaction had been completed. It would have been prudent to call and make sure which he apparantly didn't do.
I fear that you will find that it was completed and an automated e-mail sent but at least my suggestion gives you a fighting chance.
Good luck.
The issue I see with this argument is that Opodo must have received the request for the booking and the credit card information and there is no dispute that they requested the booking and that it was a valid booking
The same applies to the second booking ; given that both were made by choice by the customer and there is no dispute that the transactions were deliberately requested by the customer I cannot see how there is any grounds to request a chargeback via the card company
Proceeding without getting an explicit response on whether the booking was completed seems to be the downfall
Dave