Schroders increased its stake in the firm in May, according to accounts Benchmark filed with Companies House.
The holdings were increased to 77% on 25 May, the documents show, up from the 65% it previously held.
Schroders first took a stake in Benchmark in 2016
Schroders' half-year results, which were published in July, revealed that Benchmark Capital contributed £700m to its £1.2bn net inflows.
Benchmark Capital owns Chartered Financial Planning firm Aspect8 as well as wrap platform Fusion Wealth and Evolution Wealth.
The original business was founded by Chartered Financial Planner Ian Cooke in 1993.