The police don't allow public access to the PNC, its actually a fairly complicated database and not user friendly. This is where the provenance companies like HPI come into it, they take the raw data out of the feed and turn this into something useable.
Most of the stolen data you see on HPI etc comes from the insurance companies, its far more comprehensive and accurate than the PNC and updated far quicker, the PNC info simply fills the holes where the vehicle isn't insured.
You woudl be amazed how difficult it is to get data from the authorities. For example in ireland there is no central record of stolen vehicles available like the UK, the data the provenance companies use come from insurance companies rather than the police and in the UK there is no separate register for trailers and caravans, they are held on one huge register along with all other reported home theft item.