Budget Contingency

As I understand it, there is a "Contingency Reserve" set aside every year for emergencies and other unexpected events that may cost money over and above what is budgeted for. I read somewhere (that I cannot remember), that the contingency reserve for last year was £57,000,000,000. Which is a staggering amount. It is 10% of total expenditure, where does it go?

Does it actually get spent even if there are no emergencies?

Is there anyone out there who can bring some light on how much it is and what happens to it?