Thanks for the replies guys, I've checked the activity monitor and called sp_help_job for the specific agent job but neither show any problems. I've even looked at the job steps execution history report for the agent and that's showing no failures or issues.

The only 'out of the ordinary' event last night was that I restarted the server at 17:17 after installing windows updates but all the SQL services (including the Agent) started up again automatically and I ran DBCC CheckDB on the DB's at 17:21 as well as checking the agent jobs and nothing was amiss. The full backup also ran at 20:30 the same evening and that's a scheduled agent job as well!! Baffled much........... :ermm: