To report the company, contact Trading Standards for the are which the company is based in. (Enter their postcode here:
http://www.tradingstandards.gov.uk/advice/consumer-advice.cfm ).
To pursue your money (which is a separate issue), send a formal demand for payment to the company. Your letter must show you name and address, together with that of the company. (Ensure that you use the registered address, which might be different to the trading address. It should be shown on the company's paperwork or you can find it from here:
http://wck2.companieshouse.gov.uk//wcframe?name=accessCompanyInfo ). Your letter must also be dated and clearly state that is a demand for payment, including the amount demanded and the reasons for the demand (including details of how you arrived at the figure you've used). It must also state that if payment is not received with a specified period of time (I suggest 21 days) then legal action will follow. Send your letter by recorded delivery, using the Royal Mail's tracking facility to note when it arrived and who signed for it.
If you don't receive payment within the specified period, commence legal action online here:
https://www.moneyclaim.gov.uk/web/mcol/welcome