Just spoken with an underwriter at Covea and, there is no requirement to register the V5 of a 'keeper' to the business, for the purposes of the insurance cover. They do require 'legitimate' proof that the vehicle is owned by the business, in the form of purchase receipt with the name of the covered party.
This is the same advice as when I enquired previously.
It follows that if you personally own the 'keeper', not the business, then slap it on the Motor Insurance Database thinking it's covered.... well, in the event of a prang et al, it is not covered by the trade insurance as it won't be the property of the covered party....if that makes sense.
hope this helps