I was self employed with a company that owes me money and

Ignore the above!

Are there refusing the pay because of the quality of the work?

We can chat here.

If there is a contact, it is legally binding on both. Parties and have to pay what they owe you regardless of the quality. Unless there is a clause in the contract that says you will only get paid if you produce specific or high-quality work.

1. I would send them a formal letter before claim (you can use the template below or find other templates online)
2. -f they ignore the letters, you can start a small money claim online against them. This can be done at: https://www.moneyclaim.gov.uk/web/mcol/welcome.
3. This link has some helpful information https://www.citizensadvice.org.uk/law-and-courts/legal-system/small-claims/making-a-small-clai. The process is user-friendly and relatively easy. 

You can use this letter as a template which I amended for you:

LETTER BEFORE ACTION

Dear Sirs,

Re: Claim for (insert sum)

I refer to the above matter. (Insert details of the dispute).

I have provided services to you on a self-employed basis, and you have paid to pay my invoice dated  (date). We have a written contract which is legally binding, and you are under an obligation to pay me for my work. 

If they do not send me the payment within 14 days, I will have no choice but to issue a money claim against you for the outstanding amount. 

This letter is being sent to you in accordance with the Pre-Action Protocol (“the Protocol”) contained within the Civil Procedure Rules (“CPR”). In particular, I refer you to paragraph 7 of the Protocol, and paragraphs 13 to 16 of the Practice Direction on Pre-Action Conduct and Protocols regarding the Court’s powers to impose sanctions for failing to comply with the provisions of the Protocol.

I therefore put you on notice of my intention to issue county court proceedings against your company for my losses should I not receive payment in full by 4 pm on (insert date 14 calendar days).

Should court proceedings be necessary I will claim the court issue fee and statutory interest. Should I succeed in obtaining a judgment, the same will be transferred to the High Court for enforcement against you whereby further costs will be added to the judgment sum.

I trust the above will not be necessary and I therefore look forward to hearing from you as a matter of urgency.

Yours sincerely,

(insert name)

You are welcome. I am happy I was able to offer some help. If you have further questions, please feel I hope I was able to offer some help. Thank you so much for using Just Answer.