Anacron error emails

since i have got my incoming emails sorted i am now getting this error email to my root/postmaster account

/etc/cron.daily/apt:
/etc/cron.daily/apt: could not lock the APT cache while performing daily cron job.
Is another package manager working?
run-parts: /etc/cron.daily/apt exited with return code 1


has anyone seen it before/know what it is?

 

Is there another instance of apt/aptitude/Synaptic running? Please check with

Code:

ps aux