Last year a company for whom we did work paid us twice for the same invoice. It has gone unoticed until now. We no longer work for them. Do we have to repay (approx £500 plus vat).

So they paid you twice in error and you want to know if it's all right to keep both payments and not let them know?

Do I have this right?

No, you don't get to keep the second payment based on an error on their part.

This is not the moral board but your question surprises me.