Hi there,
When I try to delete a linked page I get this error when I submit:
one_off type cron jobs MUST have their "when" attributes set to a "OO=" prefixed value [CRON0043]
and the changes are not saved.
How do I fix this?
How are you trying to delete the page? It sounds like you're trying to set a future linking cron job.
I am on the linking page for the webpage. In the Future Linking section I select a link to delete from the list.
I didnt set a time for the job to run, I thought that it would run straight away.
I have since redone the process setting the "at" to run on the next scheduled cron run and there was no error
Yeah, you don't need to do that. 
Just click on the asset and hit "Delete" in the asset map and it'll move the asset to the Trash for you. Alternatively, you can click on the icon and move it to the Trash manually.
The Linking screen is used to schedule future link changes (including deleting links).