You must make a formal written demand for payment. That demand must include:
Your name and address ;
The name and address of the person whom the claim is being made against. (Assuming that it's a limited company, the letter needs to be addressed to 'the Company Secretary, xxxxx Ltd', not to any named individual) ;
The date of the letter ;
A clear statement that it is a demand for payment ;
The exact amount being demanded ;
The reason for that demand ;
A statement that, if payment is not received within a specified period, legal action will follow. (There is no set period but a court might rule that 10 days is too tight a period; I'd suggest 14 or 21 days).
Your signature.
There is no legal requirement to send the letter by recorded by deliver but, in case a court needs to see evidence that such a demand was sent, it would be wisest to do so.
If payment is not forthcoming, use the online claims procedure here:
https://www.moneyclaim.gov.uk/web/mcol/welcome