I've now caught up with the rest of Chris Wylie's testimony to the DCMS committee yesterday.
Major claim from Wylie: Robert Mercer investing in Cambridge Analytica allowed him to channel millions of pounds of funding in kind for campaigns (Brexit, potentially Trump) without the funding appearing in any campaign accounting.
Wylie states that some beneficiaries of CAnalytica's work were massively under-billed for it, eg £50,000 for work that had cost £ millions.
I think this is possible because of the novelty and nicheness of the work, as well as the general secrecy: a hotel or airline billing a campaign massively under cost price would immediately be flagged as giving a donation in kind. Because observers can see the goods/service being used, understand the provider's likely costs, and can also compare the prices offered to other campaigns and by competing providers.
But as Wylie says, a company developing a custom database and software from scratch and then offering it to several organisations campaigning for the same cause (Brexit) for a fraction of the cost can be hidden by describing the funding as R&D investment for the company, rather than spending on that cause.
The fact that several pro-Brexit organisations appear to have co-operated with each other and also been vehicles for getting funding and data to the Cambridge Analytica-related company Aggregate IQ for combined work (Wylie said AIQ told him they didn't segregate data or work for the different organisations, and that their work had been "totally illegal"), may be what trips them up legally.