Thanks skunk! And your suspicion is correct! Some research has indeed demonstrated an association between oversleeping and obesity. (Sources: 1, 2)
I suspect that there may be confounding medical conditions at play here, but for most people, it seems that a decent number is 7-8 hours a night. I should probably mention the oversleeping thing next time (though I'm sure undersleeping is far more common). Thanks!
cheers for getting back to me mate. There was a New Scientist edition way back in May 2016 which had loads of interesting articles all about sleep. If its online you should have a browse Sir.