It's enough to make finance directors tear out their hair. In 2002, the UK's 350 largest companies owed a collective £75bn to their final salary-style pension funds. Ten years later, after paying back more than twice that sum, the debt has barely diminished.

The attached chart from investment consultants Mercer shows how the UK's private sector defined benefit pensions - referring to old-style schemes that offer a guaranteed amount to pensioners - have remained consistently in deficit over the past 10 years.