I Worked as a sub contractor on a self employed basis and haven't been paid by the contractor.

Hi I was recently working as a sub contracted self employed contractor on behalf of a company in the security industry in the UK. No Contract was ever provided to me however the company agreed verbally that we should ideally send out invoice through on the 01st of the next month after completing work for the last month and that we should be paid by the 28th. This gives them almost 30 days to collect funds and pay the invoices, I happened to invoice late last month sending my invoice through on the 25th of March which should have been paid on the 28th of march. Because they didn't pay me on time I added on statutory interest and a debt recovery fee of £70 the company eventually paid me on the 09th of April almost 40 days after works were completed in February and they didn't pay the statutory interest or debt recovery fee. By the 14th of April they sent me an email dismissing me stating they no longer required my services without providing an explanation. I sent them an email back asking them to pay all outstanding balances as I am no longer working for them which total close to £2,500 not inclusive of statutory interest and debt recovery fees. Their reply was that I will be paid in due course without providing me an actual date that they will pay. What can I do to ensure they are actually going to pay me? They also have shady business accounts for the previous year somehow gaining almost 3/4 of a million pounds in 1 year despite being a really small firm with less than 10 full time contractors all employed on a self employed basis. Can you please advise me Kind regards Rob