First of all, you should try a few other things;
If Amazon is the marketplace and the actual retailer is different but selling through Amazon, you should obtain confirmation from them that they refunded things. Amazon may not provide you proof if they don’t have it.
Then check on your account as if the refund has been processed, surely it would show there like usual. If it doesn’t, it’s most likely not been processed.
I find calling Amazon better than emailing.
If you really want to enact Section 75 or chargeback (different things) then you will need to check you are eligible.
Was at least one of the items purchased over £100? If you bought multiple items, you can only claim for each if it was over £100 in value. You can’t claim under this section if the items each are less than £100 but combined are more than £100.
Then you will need to show substantial proof of the loss, such as confirmation that the refund took place (as above).
I am unsure off the top of my head what the Section 75 rules say about refunds not received and raising a claim for this. I would check the terms of your card in the app to read about refunds perhaps.
From experience, I still think your best route is to work with the retailer and Tymit and try to get to the bottom of this.