Command 'Calibadc' is not working on Picaxe modes 28X1 and 40X1 with Programme Editors 5.2.3 and 5.2.4. An error, 'calibadc' command not supported in this mode!' is displayed by the editor. It was OK with previous versions.
Not sure what I looked at / what I mis-read but on re-checking it appears you are both right and I was wrong; there is a voltage reference for the 28X1 and by extension CALIBADC should be supported.
We will investigate the issue. Sorry for the confusion.