I've done mine myself - I used a really easy template on the Money Saving Expert website -you just ideally need number plate / agreement number. The template can then be copied and pasted into your normal email broswer, and they tell you the relevant email to send to for the PCP company in question.
Mine (Black Horse Finance) has been accepted by them and apparently commision was involved. I'm now waiting on their further investigations, but this could take some months whilst it's all being investigated.
You certainly don't need to use a company to help you do it, it's really easy.