"built mid 70's with 5 primary schools. when i was leaving one school 1994iish (temporary contract teacher) they were struggling to stay open"
Precisely. The cycle for schools is about twenty years. People arrive on a new estate aged around 30, either with or intending to have children. Few of them are initially of school age but rapidly they are, so the schools start to fill, mostly from the bottom with composite classes higher up. After seven years the school's full and running. About fifteen years after the school opened, the people who bought the houses initially have mostly stopped having children, but have put down roots (and perhaps still have children in secondary schools nearby) so stay put. As their children leave home they perhaps do look at moving, and houses become available for a new cohort. But if the area is pleasant, a lot stay. Either way, the number of houses available to buy in year 20 is nothing like the number that were available when the place was being built.
A school large enough for the initial cohort on a new-build estate will almost always be too big twenty or thirty years later. And estates tend to have small roads and no bus connections, so the school is not well situated for people coming from other places.
We didn't see this in the past because the war had disrupted things and there wasn't a hell of a lot of choice given the massive population boom 1950 to 1965. The same went double for secondary schools: there had been no large-scale state secondary education prior to the 1944 Act, so not only did you need to cope with rising numbers, you needed to educate them to successively 14, 15 and 16 when previously it had been 12. But now, unless you're willing to write schools off after 20 years, or bus, or something, deciding where to put schools is really, really hard.
The secondary school I went to in the 1970s had a capacity, at its peak, of over 2000 (13 form, ie 390 per year intake). It was recently demolished and replaced with a bijou academy with an intake of 180, which it isn't managing to get. There are some other factors (it went into special measures, in part because of collapsing numbers, although it's now very good) but the basic problem is no 11 year olds. Other local schools haven't expanded. There's massive pressure on secondary places predicted elsewhere in the city, but realistically, keeping 1400 places "on ice" in an empty building for twenty years wasn't going to help anyone.