pgdump from 8.0.4 not valid for import into 8.1.0

[Richard Sydney-Smith <richard@xxxxxxxxxx>]

? 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

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