Thanks for that, it's kind of what I was thinking.
I also noticed while reading around the subject that the rear of the plane is where you have to buy your one reserved seat if you want your kids to sit next to you, so that would also cause a mass of people to sit at the back.
I know nothing about the physics of aviation. Does this mean that on a flight which is less full, more people have to be allocated random seats towards the front in order to balance the plane, or am I overthinking this?
I'll keep doing what I'm doing and booking 1A for the time being. I like the space, the access to the toilets, and the fact I've only got one person next to me, as well as unfettered access to the catering!