If a Londoner or "Estuary" speaker says "I been at work", it is a simple dropping of the auxiliary "have". Could be seen as dialect or sloppiness, but whichever way you see it it is the past perfect.
It occurred to me, however, that if a West-Country speaker says "I been at work", I think it may be dialect use for the simple past.
"Where were you yesterday?"
"I been at work."
Can anyone substantiate my gut-reaction?
Either way, it reminds me of travelling on a stopping train through a town in Dorset some twenty-five years ago. Pulling into the station, I heard the following wonderful announcement over the platform tannoy in broad Dorset:
"This do be Stalbridge."
Phil W. 2 November, 2004