Is centralisation of back pain from sciatica good?

Good or bad?: It is probably a good sign which means that the nerve roots are no longer irritated. Centralization of your pain with no radiation at all would mean mainly pain related to the disc which was probably the cause for your sciatica. You should still be careful with any activity as the disc might get damaged again with activities such as lifting heavy weight, sitting+bending down etc.

Specific: It would help to be more specific about symptoms.

Generally Yes: It suggests that sciatica is resolving. However if it continues you still may want to see a pain/spine specialist for more relief.