Thanks for digging that link out Minglemangle, truly eye-opening stats! The regional variation must be significant as in my locale (N London) the pupil population does not map against the same trajectory.
For instance, the 2007-08 rate is broadly similar to, or slightly lower than, 2011-12, yet when the 2007-08 lot (ie current Y6) were in Reception, our LA had to open a bulge class to accommodate them all, and in fact by the time the now Y2s had reached that stage another local primary had fully expanded to accommodate them... and then had to reduce its PAN in the second year of expansion as the places were unfilled. And the post-Brexit (presumably) fall in the rate has truly terrifying implications for school finances in the next few years. "Sought-after" schools will still be full, but the rest wil see their rolls drop and drop.
And of course it's extremely difficult for LAs to plan for these spikes and falls as although obviously they have the birth rate stats, they only find out 4 years later exactly how many of those children are still living in the area when they need a school place...
Which makes me wonder, given my DD's Y6 year is not full (though it was in Reception) - and I know her school is far from unique in this regard - how much impact will the famous "high birth year" we all remember from primary applications actually have on this year's round of secondary ones. Are all your DCs' Y6 classes full?