Refunding customers by cheque after the invoice has been paid in full

SUGGESTED

I need to partially refund some customers by cheque, they have all paid their invoices in full by Bank Transfer.

Last year the government announced hat the Clean Heat Market Mechanism (CHMM) Scheme could commence on1 January 2024.  We were charged a CHMM Boiler tax of £120 + VAT by our boiler supplier between 1 January 2024 and 15 March 2024, we then passed this cost on to our customer within their invoice.  On 14 March 2024 the Government announced that they were delaying the scheme until 1 April 2025 so our supplier has now issued us credit notes for each of the boiler CHMM levy charge. 

So I now need to refund the customers by cheque - how do I do this correctly?

Thanks