Investors wait an average three to five years to see a return but for some the long-term nature of private equity investment has stretched to 16 or 18 years.

Almost half the number of venture-backed deals have failed to exit after 18 years, according to figures collected by the Centre for Management Buy-out Research (CMBOR). An unknown proportion of these deals may have disappeared in sales back to management or other unrecorded exits, while others remain on the books.