Seal thinks "having fun" is the key to a successful marriage.

The 48-year-old singer believes his relationship with Heidi Klum - with whom he raises four children, Leni, seven, Henri, six, Johan, five, and two-year-old Lou - has lasted so long because they always enjoy the time they spend together.

Seal - who married the supermodel in May 10, 2005 - said:"We do have a lot of fun together, but you've got to. We lead such hectic lives, so we make the time we have together count.

"We're best mates, the chemistry is great and we love spending time together - family always comes first, no matter what."

The couple and their children live in Los Angeles but they schedule regular trips away.

Seal explained: "We spend every May at our home in Mexico because we do our wedding vow renewals there, and sometimes New Year too.

"We might go skiing for Christmas and we spend time in Europe over the summer then the rest of the time we're in LA."