First, from looking at your first question and reading this one, I assume you've already pandered to the clients apologies and promises of payment too much. As well as this, I'll be assuming you're set on taking court action, and will therefore address the process required before action; rather than assess whether the option is viable or worth your time, only you can decide and you seem to already have the answer.
Any further communications you send to him should formally address the issue and not the cause. The issue is you are out of pocket, and that's it really, any communications shouldn't insult or inaccurately accuse your client of anything.
I'd like to point out quickly that IANAL and most of SE's community aren't either. No one here is able to provide 100% accurate legal advice, and I suggest you consult a lawyer or legal professional for some legal guidance on the matter.
For the UK, Citizen's Advice and Gov.Uk have some great advice and guidance on taking court action, from the prerequisites all the way through to the end of the case. Most of the information below has been taken from the above sites.
You should definitely write a letter to your client before taking action, else you may end up being liable for the client's court costs. Your objective is to show and provide full co-operation and allow the issue to be resolved. If you and your client can come to a viable compromise* then take it, the court will not favour you if you are unreasonable and refuse to co-operate for the point of taking court action.
Now to actually writing the letter. From citizen's advice, it says you should include your:
- name and address
- reason for claim
- any facts that are relevant (work done, them using your work for any monetary gain, etc.)
- what you are asking for (make sure this bit is as clear and unambiguous as possible)
- how much you're claiming (including any court costs you wish to claim back and any expenses with sources)
- a list of documents you'll be providing your (signed contract(s), email strings, invoices that have been sent, etc.)
- A deadline of acknowledgement (14 days) and payment (30 days) (these seem to be considered reasonable enough time frames)
- A reminder that you will be starting court action if the payment is still outstanding by the time of the deadline.
- A reminder of rules set within the Civil Procedure Rules. (I don't know enough about these rules to give decent advice so give
that link a read for more information.)
Finally, attach an Invoice to the letter as well, get proof of posting from the post office, and keep a personal copy too. If you haven't received a response or payment within the given deadlines, move towards starting your claim.
* and make damn sure it is viable, as you may not be able to back out easily upon accepting it, and I doubt any court would take your favour if you've accepted a compromise, only to back out and still claim the money.