Hi,
Hope some of this helps -
"We did not get 1st choice at primary intake. I live 50 metres from catchment boundary, and, for whatever reason, was mistakenly told some time ago to apply to this 1st choice school regardless. I bought my flat based on the fact it was in this catchment and now, finding it has either changed this year (i need to find this out) or I was always just outside it, means I would like to move, permanently, closer to the school, well within the catchment"
re catchment
Catchment area is a myth - any school's 'catchment' will change every year depending on the number of people who apply and according to the criteria. E.g. if (as most schools do) main criteria is sibling priority, then the places will go to siblings first no matter where they live. Then the remaining places typically go according to distance (depending on criteria specific to the school - e.g. faith schools have different criteria). If there are 20 places left after siblings have received their places, then those 20 places go to the people who've applied according to how close they live to the school. If all 20 live within a 50 metre radius then that is the 'catchment' for the school for that year.
re applying to your preferred school
I think you should always applied to your preferred school. Not sure exactly how this works, but you will stay on the waiting list for your preferred school even if you start at a different school - so you should always put this first. I'm not sure if, say you get your 6 place school, and you'd be happy with EITHER your first or second choice, and you think you have a better chance of getting into your second place school - I'm not sure if you can be on the waiting list for your second place school.
If you are 25th on the list, it might be worth starting at the school you've been offered and staying on the waiting list for your preferred school? Lots of people do this and I think kids (esp in London) are used to moving / new people joining schools so it's less of an issue than you might think it would be.
I'm pretty sure Papinian is wrong (shock horror!) if you move closer, and you are genuinely living there, then that's fine. You will have to stay there for a while at least otherwise it would be strange but I think you are genuinely renting out your flat and renting a new flat (and not moving back within 2 weeks) then that is fine (though probably frowned upon!)
What school have you been offered? Might be worth asking views on that school specifically - it's very difficult to assess a school in a 1 hour tour so worth asking for views....
Good luck!