Write to your bank / card provider and inform them that you are making a claim under Section 75 of the Consumer Credit Act.
Give them clear details of the transaction – the name of the business you paid, the date you paid and the amount you paid.
Advise them of the reasons / grounds for your claim – i.e., promised service not provided, misrepresentation or the bankruptcy of the business you paid. Provide them with details that support your claim.
Request that they acknowledge you claim within 14 days and at that time, give an indication as to the time they will take to assess it.
Remember that Section 75 claims are a part of the laws that protect consumers and so your bank is obliged to respond.