Ok here is more info... and it gets strange.

I rebooted the SQL server (ugh, I don't like doing that)

Tried a couple of jobs and they ran fine and assumed things were all ok.

However this morning a couple of other jobs failed with a 7340 error (

Cannot create a column accessor for OLE DB provider "IBMDASQL" for linked server "linkname".) instead of 7302. Further research showed that those using openquery() succeeded, where as those that simply queried the linked server such as below, failed with the 7340 error (At the time these jobs were created I was unsure if there was a performance difference with the openquery() method. It does not appear to be any significant difference)

FROM Linkname.servername.catalog.table AS derivedtbl_1

I also noticed in the history that these jobs had first failed with 7340 before failing with 7302.

I did some searching and found this link:

https://connect.microsoft.com/SQLServer/feedback/ViewFeedback.aspx?FeedbackID=277518&wa=wsignin1.0

where a user points out that simply opening the properties on the link 'fixes' the 7340 error. I opened then closed the properties window and the 'broken' jobs started to work. Obviously there is something going on here, but it is a bit mysterious.