Your report time should be the time you leave one base for the other. For example if you are MAN based and operate a flight from BHX and are required to self position then your on duty time commences at the time you report at MAN to position to BHX. Your off duty time is when you get back to MAN or if you have post flight duties at MAN then after these have been completed. Having said this your flight duty limit is based on final chocks at BHX, after final chocks you can position for hours more but your minimum rest must reflect this period.
Hope this helps.