A simple letter emailed and a hard copy posted along with a copy of the invoice and clear payment details. The letter should state 'final notice for payment' then simply request payment be made in full for the attached invoice within 7 days other wise you will submit a claim to the county courts this will also incur charges and interest.
If it still is not paid after the 7 days expires simply go online to your country court and issue a claim against them add on the £25 you will have to pay and 8% of the debt under the interest of commercial debts act. Submit the claim then sit back and wait either for a response or payment.
If the debt was above £500 you could use a collection agency which usually collect on no win no fee and only charge around 20% of the debt once collected. But for £300 its not worth it just issue a country court claim against them.