Hi Andrea, thank you for your help and for asking to look at the flight logs, it was called "Copy of copy of Surrey Sawmills" the date says Jan 30 2018, third one from the top on my dashboard.
Just to clarify, I flew the mission grid/nadir photos, all went well, it was the "3d mode" that only worked for half way round the area. My battery needed changing to finish the 3d mode perimiter flight, but after I swapped batteries, it wouldn't let me continue. It went through the check list but stopped on the last one, "Flight Plan", it said it couldn't load for some reason. I flew the rest of the perimeter manually via DJI Go. I managed to nearly fly the perimeter, but then dji Go froze on me, then crashed completely. Lucky I aways maintain line of sight and flew back manually, so no insurance claim needed this time!!
I hadn't updated the P4Pro to the latest update as I had heard of issues with this one and the drone had been flying faultlessly up to then. I got home and have done the update, will go out to re-calibrate and test fly as soon as possible.
Thanks again, look forward to hearing your thoughts on the findings from the logs,