urgh! this problem got stranger and stranger. so i cancelled my NAS job expecting my regular job to start. no, that then gave the same message about waiting for tape resources. what!? its a different job, different pools, etc. so i stopped that job. waited for it to mark as failed. then started it again. same message - it was waiting for a tape from a drive that job had never referenced before! i thought 'great, all my tape jobs are hosed!'.
i reviewed all the settings then recalled that when Veeam support were remotely connected on friday they changed my regular backup tape job to target a different pool (this was to attempt a fix an issue in #01927063. thankfully i have now reverted the settings and my regular job is working as normal.
Grrr to Veeam support. i think i'm entitled to a free upgrade to Ent Edition for this inconvenience
ps: sorry fir the ranting first post - i hope you appreciate how baffled and confused I was.