According to Wikipedia it's 15%
http://en.wikipedia.org/wiki/Richard_Branson
`On 4 July 2006, Branson sold his Virgin Mobile company to UK cable TV, broadband, and telephone company NTL/NTL:Telewest for almost £1 billion. As part of the sale, the company pays a minimum of £8.5 million per year to use the Virgin name and Branson became the company's largest shareholder.[citation needed] The new company was launched with much fanfare and publicity on 8 February 2007, under the name Virgin Media. The decision to merge his Virgin Media Company with NTL was in order to integrate both of the companies' compatible parts of commerce. Branson used to own three quarters of Virgin Mobile, whereas now he owns 15 percent of the new Virgin Media company.'
He should change his name to Richard `The C$sh' Branson.
8.5 million a year minimum for the use of the Virgin name. That's only 19142 customers on full price XL broadband per year (I think it's still £37/month for XL). NTL/Telewest should have keep the extra cash and just incorporated Virgin mobile into their company name, as they were already known as a landline supplier and adding mobile services would have been a natural progression. Then renamed to something like NTL Media