The tycoon’s complex finances are under scrutiny as the NCA investigates £8m of Brexit campaign funding

Arron Banks is at the centre of a National Crime Agency investigation following the Electoral Commission’s report last week that it could not be certain that he was the true source of £8m given to the leave campaign.

Banks, who is principally known for his insurance business but holds interests in other areas, insisted in a round of media interviews on Sunday morning that he was the source of the money. But questions have been raised about Banks’s wealth. Some of his companies appear to have generated either losses or only small profits, and offshore secrecy makes it difficult to assess the financial health of others.

Best understood are his interests in insurance. His company Eldon Insurance Services, which operates under the trading name GoSkippy, is the principal vehicle of his insurance group. In addition to selling its own cover, it has other revenue streams, including a partnership with Debenhams.

Eldon has generally turned a small profit over the past years, making a profit of £1.9m in 2017 and a loss of £26,000 in 2016. It is owned by ICS Risk Solutions Ltd, another Banks company based in the Isle of Man. The investigative website Source Material has suggested that it may have at least one undisclosed shareholder besides Banks. Source Material said Banks did not answer its questions about the shareholdings.

The second key element to Banks’s insurance empire is Gibraltar-based Southern Rock, Eldon’s principal underwriter. Questions have also been raised about Southern Rock’s financial health. Earlier this year the Financial Times reported that it made multimillion-pound losses in five of the past seven years, including a £32m loss in 2016.

In 2011 the company’s capital base was “technically insolvent”, according to its accounts. The problem was ultimately solved by selling rights to certain future profits to ICS Risk Solutions for £77m. ICS Risk Solutions’ accounts are not public.

Crucial to Banks’s insurance group, as well as the controversy surrounding his Brexit donation, is Rock Services. A UK-based recharge services company, it invoices his other companies, with expenses closely tracking revenues, and generates little profit itself.

Banks originally told the Electoral Commission that he personally had loaned money to his pro-Brexit campaigning group Leave.EU. However, the commission concluded earlier this year that the money had actually come from Rock Services.

Complicating matters further is Rock Holdings, an Isle of Man-based holding company for various strands of Banks’s insurance business. According to the Electoral Commission, Banks’s business associate Liz Bilney told investigators that an £8m shareholder loan to Banks, which financed his Brexit donation, was recorded in the accounts of Rock Holdings, despite having been made by Rock Services.

Bilney insisted on Twitter that the arrangement was permissible.

Separate to the apparent struggles of his insurance business, Banks has suggested that his wealth could be attributed to his sale of shares in other companies. He has previously responded to questions about his wealth by tweeting that he founded and sold Brightside, another insurance company, for £145m.

However, the website OpenDemocracy said its examination of company documents suggested that Banks had made substantially less than this from the sale.

In 2014, a company in which Banks had held shares, NewLaw Legal, was sold in a deal that valued it at £35m. However, Banks had offloaded his own shareholding 18 months before the sale, according to the Financial Times, which estimated the value of his shares at £1.75m.

More exotically, Banks holds interests in diamond mines, and in 2014 he bought a Bristol-based jewellery business called Parsons, which sells diamonds sourced from diamond mines that he owns in South Africa.

Although the precise ownership structure of the mines is not clear, documents in the Panama Papers revealed Banks to own a number of entities that may be related: Precision Risk and Intelligence (private intelligence); African Strategic Capital (wealth management); African Strategic Consulting (lobbying); and African Strategic Resources, which holds mining rights.

All four entities were owned by a British Virgin Islands company called PRI Holdings Limited, itself majority-owned by Banks. None of the relevant financial accounts are public and Banks could have since changed his affairs.

Earlier this year, Channel 4 News obtained emails showing that Banks transferred thousands of pounds to a cabinet minister in Lesotho at the same time he was applying for a mining licence. Banks has denied bribery, and the minister said the payments were support for political and charitable activities.

As of earlier this year, Banks also held around 30% of the shares in Manx Financial Group plc, an offshore services firm. Investor statements reveal that Banks owned the shares via Marquise Holdings Limited, reportedly based in Gibraltar, though the Guardian has not been able to locate this company. The shares were transferred to ICS Risk Solutions earlier this year.

The same statements suggest that Banks’s assets may be partly held in trust, describing Marquise Holdings as “an asset of the Banks family settlement”. No other publicly available information about this settlement or its activities can be found online.

An interim report by the Information Commissioner’s Office on possible data-sharing between Banks’s insurance and political interests is due to be published on Tuesday.