pgdump from 8.0.4 not valid for import into 8.1.0

? In pgAdminIII linked to 8.0.4 server did a backup of all databased.
Chose "plain" and saved to disk.
uninstalled 8.0.4
installed 8.1.0

create database in pgAdminIII v1.4
restore
---- reports as not a valid archive.

Obviously this is not what I expect.
Some older backups from a 7.4.? server on a Redhat machine will restore.

Please ? why is what I have done failed.

thanks

Richard