If you have exhausted your resources in terms of sending reminders and late payment requests with no result, there are a few alternatives that can be taken before embarking upon legal action.
One step you can take is to involve people higher up in the customer organisation, such as the finance director or any other relevant senior staff who might be able to influence the quick payment of the debt.
Additionally, you could emphasise a longstanding and amicable relationship where relevant to do so. State that you have supplied a high-quality service or product and you wish to maintain the mutually beneficial and friendly relationship you have had with the customer up to that point, and this may do the trick.
However, remember that there is only so long you can wait for payment to be made – provide an escalation date in your customer agreement and use it as described, whether it involves a debt collection agency and/ or a solicitor. Whatever the agreement, stick to it.