You can’t tag one bank transaction as a payment from more than one client, but with something like GoCardless you should already be using a holding merchant account (since they deduct their fees before sending you the money). So when money is deposited in your bank account you treat it as a transfer from the GoCardless merchant account. The actual invoices would be marked as paid individually into the same merchant account, and the difference (the GoCardless fee) would need to be allocated to a purchase.
http://community.quickfile.co.uk/t/handling-payments-from-merchant-accounts/8865