Dear All,
I think that I have found a workaround. I found another server with Patch installed and working and:
Copied the AexPatchAssessment.exe file into the following directory:
..\Altiris\Patch Management\Packages\WindowsVulnerabilityScan
And ran the Package Update Distribution Points task on the server.
The agents then got their download and all was OK.
However, what do we do if there is not another, handily-placed server available?