Hi there,
You MUST follow a certain path, in order to get justice for yourself. The law will not be that slow and I am very sure that, if his business is that 'dodgy', he will not wish for the magnifying glass to be put on him.
Send an email that politely spells out your complaint and entitle it 'Grievance'. Be as clear about your complaint(s) as you can and spell out what will remedy it - i.e. to receive your due holiday pay and any other monies that you feel that you are entitled to.
State that you have been in contact with ACAS (make sure you ring them to reassure yourself!) and that you hope he can resolve this matter very quickly and allow you all to move forwards.
Try this link to read your rights and the grievance structure:
http://www.acas.org.uk/media/pdf/s/g/acas_Disc iplinary_folder_1.pdf
Once he is in receipt of the 'Greivance', he has to reply within 28 days. I am sure that he will resolve this when he realises you are serious. Ensure that you send a hard copy by recorded post, in order that the email document service cannot be denied by him at a later stage.
The fact he is an ex-RMP is of no consequence and it would not matter if he was the serving prime minister, he cannot avoid the law. I am guessing he is an intimidating character and I understand that this may make you not relish tangling with him.