My phantom 4 was hovering for about 1min when it randomly shot forward and hit a building roof. The camera glass has cracked.

The remote was in the floor so I could get something from my bag. Can't work out why it shot forward, approximately 10m

In addition, the gimbal struggles to calibrate, it will only do so if I gently manipulate it with some assistance form
My fingers.the drone still flies fine and the camera works from the remote, once I have manipulated to set it before take off.

How/where can I get it fixed?

try to calibrate it again from the remote after the initial calibration. (this setting):


Also, id recommend you calibrate your IMU if you haven't already

Sorry for your mishap. If hovering and it randomly shot forward with no  stick input does not sound right. If you play back the flight in the flight records section and tap the RC icon it will bring up the sticks and you can see exactly what happened.
And if a gimbal calibration does not remedy it, then the gimbal has been damaged and needs to be repaired.
As for the glass, the lens cover unscrews and you can put another one on.