Easy..First decide your strategy of future relationship with the client, also know their culture, to a lot of my clients , including european export, it wouldn't make any difference..yes (a ) raise a credit note, make a new invoice, and send BOTH with a friendly letter. Your question implies you don't have a relationship, that is better than ' Arms Lenght '..and the relationship ( unless this is a one off opportunity ) remains all about the future, and using your inter personal skills..the latter really is what it's all about.UK ltd, does not prevent you from raising a second invoice, plus a statement to account, listing both invoices, and terms of trading, possibly asterisk against first invoice on the statement, advising this is due for payment,..did you do a credit check, and do you have a policy of Indemnity exposure insurance..getting more difficult to achieve in current business climate.