The population is available online but is a bit of a search to find. Using this website
https://census.ukdataservice.ac.uk/get-data/aggregate-data go to
Casweb and click on that.
In the next page start Casweb
Select the year you want
Then on its self explanatory what boxes to tick
When you get to the end you will get figures. Note for 1981 Great Hallingbury is split in two and in 1991 into three.
The population data (Great Hallingbury) is
1981 census All present residents
Zone ID [81sas010001] (Census reference only)
23HWAH01 510
23HWAH02 378
Total 888
1991 census All present residents
23HWFH01 513
23HWFH02 336
23HWFH03 109
Total 958
Note I have only given the figures for all present residents. If you look at the data you will see they have ways of varying it
Note As these figures appear to show a fall in the population from 1971 it is possible the criteria for defining Great Hallingbury in 1971 was different to that used in the 1981 and 1991 censuses (1961 was 1001 and 1971 was 945 according to the blogspot). Of course it is possible the population did decline from the 1960's onwards and then started to increase again somewhere around the 1980's. You would need someone with local knowledge or to contact the Council as mentioned above to find out.