Premium Bonds. Always a winner.
Except when you don't win. You effectively loose money.
He's basically guaranteed to win! He can't lose!

The only way the OP will lose money is if he doesn't have average lucky - check out the calculations.
http://www.moneysavingexpert.com/savings/premium-bonds-calculator/
Considering he's just come into £5k for what ever reason then that's rather lucky, so it's safe to assume he has more than average luck. He's basically guaranteed to win! He can't lose!
Always pay off your debts first. The interest you pay will almost always be more than the interest you receive.