Thank you chaps, especially those who expanded the sums to digestable chunks...........I was going backwards and squaring the speed to double the lift (requiring 12,100 knots to get airborne at 400,000 kg - which I KNOW is absolutely not true cos I've been there - 400,000 kg not 12,100 knots !!!)
Obviously the birds haven't worked it out either 'cos I don't think there's a bird that can double its weight and still fly ?