Keeping seller's money in order to gain Interest

Company sells cars on behalf of Private Owners wanting to sell their car.<br> Sold car and registered on 15 June 2015 on our behalf. Finance company paid over money on 29th of June to them of which I saw the proof of payment. Up to date 2 July they have not paid money over.<br> They claim all things regarding that they do not have internet to see if they received money etc, money is paid over to another company and then later only paid to them etc etc and they need to keep it 14 days before paying it over.<br> It seems that they want to earn interest as a bonus on the deal.<br> When phoning them , they never get back to you, put the phone down on my wife when we do enquiries. At the end it is all a lot of promises with no action.