In case the past due invoice emails still do not work, here are some options for trying to recover your money;
This is a process that tries to resolve conflicts without taking things to court. There is usually an impartial person that acts as a referee in the dispute, and they aim to try and reach a resolution that works for all parties concerned.
A statutory demand is issued when you wish to place a demand on the client to pay the invoice.
In the UK, there are many different websites where late invoices are reported. These include:
This website was designed in 1997 by the UK Government, which partnered with various business organisations that were offering SME services. Its primary function is to help UK businesses get paid for their services.
If you have an overdue invoice, then you are likely to find your name on their website. Most businesses have already registered on this site, and they send information about unpaid invoices so that others can see them before getting into business with you.
The Business Disputes Register 
This is yet another website that seeks to resolve any conflicts between businesses, including unpaid invoices, which means that you may be reported on the site if you haven’t paid your invoice.
They allow businesses to raise disputes which in the end help erring customers pay up their invoices. Once you pay, the information can be removed, making your record clean.
This website helps businesses report their experiences on unpaid invoices. They have an online directory that is very simple to use, and any creditor can post your name to show that you did not pay their invoice.
All the information is made public, which means that anyone can see your information and payment history with a said creditor.
If the client still doesn’t pay the amount they owe, it’s probably time to take legal action. But, the good news is that; small businesses do not need to get a lawyer at this time.
The UK law allows anyone who is owed money to make a statutory demand for it, but this must be done before the debt exceeds six months, and it must be done in person.
Once the client is served with a statutory demand, they are required to either pay the overdue amount in the next 21 days or agree on how the money will be paid.
After the 21 days?
Now, if the 21 days lapse and payment has still not been made, you have a few options;
If the amount you are owed is less than £750, you are allowed to apply to wind up or bankrupt their company, although this has to happen within the next four months of the statutory demand.
If the amount is more than £5,000, you can submit a petition for bankruptcy to the courts.
It is wise, at this point, to bring in a third party to handle the issue for you, such as a third-party mediator, a solicitor, or another advisor.
In case the amount is above £10,000, your case will be submitted to Money Claim Online. 
This is an internet-based HM Courts & Tribunals designed to solve debt collections without necessarily going through the courts.
The client can also choose to take you to a debt collections agency that will handle the matter on their behalf. This agency usually collects debts for a fee.
Documentation needed for taking legal action
The first thing you must ensure is proof that you tried to resolve the problem before taking any legal action. If you have not done this, then the court may go against you.
It would be best if you had the following; Letters, emails, and any other recordings showing that you did try to either use mediation or negotiate with the client.
You can write the initial letter on your own, but it is always advisable to have your solicitor do it for you, as the client is most likely to respect that more.
The solicitor will also ensure that they include all of the information required for your specific situation for the court if you decide to go to court after all.
How to make the money claim
You can make a claim in court for the amount of money you are owed. This is often referred to as “a small claim” or “a money claim.”
This amount can include any interest you have charged the client or compensation you are owed.
The customer may pay as soon as they receive your claim to avoid getting a court order against them. If they do, it will not cost you any more than the original fee, and you will not have to go to court.
If the money owed is a lot, then the situation may become complicated, and you are less likely to receive payment immediately. The claim may go to court if the customer continues to disagree with your claim. This is pretty expensive in the long run.
Also read: What to do if clients do not pay their invoices on time