Protected refund via Mastercard?

[Amilcar]

I recently ordered goods over the internet which were not delivered. Eventually I cancelled the order and was promised a refund to my Mastercard. There were several further promises but no refund. I am obviously being given the runaround by the supplier. Contacted RBS Mastercard helpline and was told I was automatically covered by them. However, I fear there may be a snag. The company I bought from said they accepted Mastercard and I filled in my card details at their website "check out" but when confirmation of the order arrived it was from a firm called "Protx" and/or "SagePay" (which apparently processes payments and is a "third party service provider." ) Have read Consumer Credit Act section 75 (with regard to joint liability) and worry that involvement of this party might bar me from an 'automatic' credit card company refund. Have put my concerns to the customer service assistants on helplines at both Mastercard and Protx and both say that Protx/Sagepay are only administrators and I will be protected. But they may not appreciate the law and the loophole. Would value legal opinion/advice, pls?