Occupational pension schemes are not run directly by your employer. Instead they need to be run by a board of trustees and their finances kept entirely separate from the employer's finances.
The trustees will usually be made up of a mix of senior managers and representatives of scheme members. Trustees have specific legal duties – management trustees should not simply act in the interests of the firm and member trustees cannot simply be delegates from the workforce.
The trustees will have specialist advisers to help them, and will usually delegate day-to-day administration to a specialist company, though some big companies run their schemes internally.
There are many legal requirements on pension funds and their trustees. They must obey the many rules set by HMRC, and comply with all guidance from the Pensions Regulator. You can find out more about what trustees do on the Pensions Regulator website.