The nation's happiest 10% of people have at least £310 a month as spare cash, nearly double what the average Joe has to enjoy.Research into the correlation between spare cash and happiness found that on the average UK salary of £23,640, 81% of money earned is spent on essential outgoings.That means that just £1 out of every £5 we earn is spare cash, or to put it another way, if you work five days a week, then the money you earn from Monday to Thursday is already earmarked for your regular outgoings.The research showed that average monthly income in the UK is £1,970. After rent or mortgage, loan or credit repayments and utilities are paid the average spare cash per household was £381. That becomes £166 per person per month, or £38.19 a week."Our research shows that having spare cash - money that you haven't already got earmarked - is really important for happiness. It shows how beneficial it can be to budget, to try to give ourselves a bit left over at the end of every month," says Dean Lamble, managing director of SunLife."It also shows that financial happiness could be more attainable for many people than they might think, with the average person needing to find an extra £33 a week - less than £5 a day - in spare cash to be among the happiest in the UK."The research found the Scots among the most flush, managing to set aside £454 per household every month.High living costs pushed London near the bottom of the happiness index as its residents only managed to scrimp together a spare £350 per household per month.Meanwhile the least happy were people were resident in the East Midlands, managing to set aside £215 spare every month.However, one rather mean spirited result found that those who thought they had more spare cash than their peers were happier than those who believed they had less.