Given the recent weather i have had some great fun in the bongo! 4 wheel drifting and dognuts

At idle stationary i have no power steering. When i rev it to 1500rpm power steering works. Its parked up atm but is my daily so i really need some help please..
I have checked the belt and fluids all ok. I have oiled the uj in the steering arm...
How do i determine if its a steering pump or rack issue?
Its a n reg diesel
Help advice would be appreciated